How does one effectively understand & navigate through the suicide cases found in the canon?

Piya Tan provides a lengthy analysis for this topic in his commentary to Channovada Sutta (MN 144).

He says the traditional commentaries maintain that the monks who committed suicide were not arahants when they took their own life, but became arahants at their final moments.

On the other hand, other modern day translators and scholars opine that according to the literal wording of the Pali version of this sutta, it appears that Channa was indeed an arahant when he committed suicide.

Some argue that it may be possible for arahants to commit suicide to escape extreme physical suffering from severe incurable illness without experiencing mental aversion. Meanwhile, others argue that the first precept and the third parajika rule for monks do not allow the taking of a life, even one's own life, hence it is not possible for arahants to take their own life.

Despite the controversies surrounding this topic, Piya Tan ends the discussion fruitfully with these statements:

7.1 Buddhism is opposed to suicide for various reasons. In a footnote of his paper, “Buddhism and suicide: The case of Channa” (1996), Damien Keown lists the following as some of the reasons why Buddhism might be opposed to suicide, which I list in full:

(1) It is an act of violence and thus contrary to the principle of ahimsa [non-violence].
(2) It is against the first precept [against harming life].
(3) It is contrary to the third pārājika [against a monk taking life]. (Cf Miln 195).

(4) It is stated that “Arahants do not cut short their lives” (na ca arahanto apakkaṁ pātenti). Miln 44; cf D 2:32 :: DA 810 cited in Horner (Milinda’s Questions 1:61n). Sāriputta says that an arhat neither wishes for death nor wishes not to die: it will come when it comes (Tha 1002 f).

(5) Suicide destroys something of great value in the case of a virtuous human life and prevents such a person acting in the service of others (Miln 195 f). Wiltshire states that altruism is also cited in the Pāyāsi Sutta [D 23,12 f/2:330-332] as a reason for not taking one’s life (1983:131). With reference to the discussion here (D 2:330-332) he comments, “This is the only passage in the Sutta Pitaka in which the subject of suicide is considered in the abstract, and even then obliquely” (1983:130). [Kumāra] Kassapa states that the virtuous should not kill themselves to obtain the results of their good karma as this deprives the world of their good influence (D 2:330 f).

(6) Suicide brings life to a premature end. As Poussin (1922) expresses it: “A man must live his allotted span of life... To that effect Buddha [sic] employs to Pāyāsi the simile of the woman who cuts opens here body in order to see whether her child is a boy or a girl” [ie to let things take their natural course and the practice of patience] (D 2:311).

(7) Self-annihilation is a form of vibhāva-tanhā [craving for annihilation].

(8) Self-destruction is associated with ascetic practices which are rejected since “Buddhism had better methods of crushing lust and destroying sin” (Poussin 1922).

(9) There is empirical evidence provided by I Tsing [Yijing]. Poussin notes: “The pilgrim I-tsing says that Indian Buddhists abstain from suicide and, in general, from self-torture” (op cit).

(10) As noted [in the Chann’ovāda Sutta, M 144.6 below], Sāriputta’s immediate reaction is to dissuade Channa, in the strongest terms from taking his life. Sāriputta’s reaction suggests that suicide was not regarded among the Buddhist senior disciples as an option even meriting discussion.
(Keown 1996:29 n55)

7.2 Suicide is clearly against the most basic of Buddhist values, namely, that of life, as enshrined for example in the very first precept and the third pārājika Death is one of the fundamental aspects of the most basic level of suffering (dukkha,dukkha). The first noble truth also implies that death is a problem, not a solution. The cause of the problem lies is the second noble truth, that is, craving (for life, bhava,tanhā, or for death, vibhava,tanhā) and the solution is the giving up of the desire for both.

What is significant is that through the affirmation of death he has, in his heart, embraced Māra. From a Buddhist perspective, this is clearly irrational. If suicide is irrational in this sense it can be claimed there are objective grounds for regarding it as morally wrong.
(Keown 1996:31)

7.3 When discussing the question of suicide in early Buddhism, especially in regards to the death of Channa, of Godhika, and similar suicides, they should be seen as a cultural phenomenon, and not as a Buddhist doctrine. For it is evidently clear that Buddhism does not condone suicide, as clearly stated at the beginning of this essay. The deaths of Vakkali, Channa, Godhika and the unnamed 500 monks at their own hands are personal decisions, even exceptional cases, and not the rule.

Although Vakkali, Channa and Godhika die as saints, their preceding state of mind that compels them to take their own lives are not an awakened state. It is most important to understand that this preceding state of mind is not the cause of their attaining sainthood, although they are the preceding condition. It is like a ripe fruit that is cut off with a picker’s knife, but neither the knife nor the cutting is cause of the ripening of the fruit. On account of their spiritual cultivation, their last thought-moment is clearly free from the unwholesome roots of greed, hate or delusion, so that they die fully awakened and liberated.

7.4 In closing this brief but exciting examination of the Buddhist attitude to life and death, we should recall Sāriputta’s saintly statement in the Theragāthā:

I delight not in death, nor do I delight in life;
I shall cast aside this body fully aware and mindful.

I delight not in death, nor do I delight in life;
I await my time as a servant his wages.
(Tha 1002 f)

Piya Tan's commentary to Channovada Sutta (MN 144)

I want to add a simile

Suppose there is a fire burning high with flames and red hot iron at the base & coals.

A man desiring the fire to get extinguished one would throw a modicum amount of wet cow-dung on it such that the flame would go out but a new flame would arise when cow-dung had dried out.

Here desiring extinguishment one only added fuel to the burning. This is akin to a Sotapanna laying down the body desiring to escape suffering.

Suppose you have only a small residual fire burning and were to throw throw a lot of wet cow-dung on it such that it would become completely cooled.

This akin to an Arahant laying down the body to become extinguished.

Liberation means that we have freed ourselves, but freedom (in the sense used here) is a subtle and difficult concept.

Suicide is vexing because those who attempt suicide (generally speaking) are working from within deep attachments: shame, anxiety or fear, despondency, frustration, or any of the states that arise in us when deep-seated cravings go (seemingly perpetually) unsatisfied. In such cases the urge towards self-destruction is actually a call for radical change, and the best course for those who feel such an urge is to turn to the dharma and free themselves from the compulsions (and sufferings) of craving. That change goes in a direction the thinking mind can neither easily understand nor easily accept, of course, but it is the change that's needed.

However, someone who has achieved liberation isn't subject to these attachments. They are not suffering from an unrequited craving; they are not despondent because the world hasn't given them what they want; they do not act out of shame or fear, having left those qualities behind. Someone who has achieved liberation does not feel as though they have left anything undone, and while they could always do more, they recognize that there is a limit to what any human can do. Then this becomes a simple, matter-of-fact decision. Can we offer something of ourselves to others, or are we a drag on their (spiritual) lives? Do we spend our waking hours in joy, or is each passing moment clung to like the pained ticking of a decaying clock? There is the risk that the agony of protracted illness might cost us our equilibrium, pulling us back into the cycle of rebirth even at the edge of nirvana...

The time to leave this body comes to all of us. Most people fight it tooth and nail; some people rush into it in a frenzy. But someone who has achieved liberation is free to choose wisely.

It's very simple: killing human is a downfall transgression. Simple getting pc3 read and understood as found in Vinayas Vibanga, althought the story of origin is as well found in the Suttas where fools killed each other, called for, suggested, killed themself. No need to see them as part of those going well at all.

Obviously they were cases of euthanasia, since the monks were elderly with physical bodies afflicted by uneasing physical conditions.

The Buddha declared Channa, Vakkali and Godhika were all blameless &/or extinguished. They were blameless &/or extinguished because they did not cling to life as "self" nor did they long/crave for another life.

Whether or not they were Arahants is not relevant. What is relevant is there was no clinging.

First of all you are equating the breakup of an Arahant's body to the breaking up of the body of a non-Arahant, the two are not the same.

Death is spoken of a as a requisite preceeding [re]birth. There is no birth for an Arahant, this is a difference. An Arahant doesn't die, with the breakup of the body they become completely released from all modes of being.

You are also slipping by the fact that Arahants are more or less the same in mind, different only in body.

It's not like one Arahant is depressed and kills himself whereas another is more psychologically resilient.

As i see it, Ven. Channa used the the knife because he considered that his being there wasn't needed as he was gravely ill. He had done his work here and had every right to end it as he sees fit.

In some circumstances Arahants do choose to go early but that is as far as it goes.

As for Ven. Nanavira and Ven. Samahita, i have little to no interest in reading works of disciples and will probably never know what Nanavira wrote in his notes. I just think reading the words of the teacher is superior that's all.

I was sympathetic to Ven. Samahita due to him always taking the time to pay homeage. Whether he was this or that doesn't matter to me as even if he was an Arahant it is of no help to me now.

As to controversies, imo the Sutta are explicit enough for anyone to make up their own mind but of course no matter how explicit texts are people will bend over backwards to maintain their presupposition. These suttas are imho so clear that i think 100/100 trained logicians will interpret them in the same way when using rules of inference and epistemological razors.