Does 'kusala' (skillful) also mean 'wholesome' and 'morally good'?

• Wholesome - based on actions who's result we perceive as desirable when experienced. They have non-greed (alobha), non-hatred (adosa) and non-delusion (amoha) as the motivating volitional factor behind these action.
• Un Wholesome - based on actions who's result we perceive as un desirable when experienced. They have reed (lobha), hatred (dosa) and delusion (moha) as the motivating volitional factor behind these action. E.g. if you take something out of attachment you will have to experience the pain of parting with something your are attached to.

There are sometimes when there is a distinction make between Skilful and Wholesome and Un Skilful and Un Wholesome by later teachers and in conventional usage.

• Skilful - actions to progress in the path with a view to attain Nirvana. These are wholesome also but some action may give good results but may prolong your stay in Samsara. Say you have strong Karma to gain wealth but this can lead to attachment and sometimes may become an obstacle.
• Un Skilful - action which prolong existance Samsara or non conducive or pose as obstacles towards attaining Nirvana. Though there can be a distinction made between Wholesome and Skilful there is not distinction between Un Skilful and Un Wholesome, as anything Un Skilful is Un Wholesome and anything which is Un Wholesome is Un Skilful also.

See: (Akusala,mūla) Añña,titthiya Sutta, Cetanā Sutta 1-3, (Kamma) Nidana Sutta, Maha Kamma,vibhaṅga Sutta

Conceptually, Buddhist ethics is very simple and elegant, at least in my limited understanding.

The state of Buddha (Nirvana) is taken as the gold standard of all the best.
While Samsara, and within Samsara the hell realm particularly, with its absolute dukkha, is taken as the worst possible condition. Psychologically, subjectively, Hell is defined as state "everything is wrong" -- and Nirvana as "everything is right"¹

The good and bad then are defined in relation to these two extremes. Whatever is conducive to psychological suchness is considered good (kusala). And whatever is conducive to psychological wrongness, to staying in or getting deeper into Samsara, is considered bad (akusala).

So in Buddhism the connotation of kusala/akusala is not a moral quality but the effect in terms of propelling us one way or another. Researchers of pre-Buddhist sources suggest that kusala originally meant "wise", "clever", "intelligent", "expert" -- and later "blameless". As in, if you act wisely, you won't be blamed. My favorite choice of translation, is one that Thanissaro Bhikkhu settled on: "skilfull". On one occasion I translated akusala as "pathological" -- kinda the opposite of "healthy", which is another connotation of kusala. Although in informal speech I simply say good (as in "drinking milk is good for you").

To add more depth and color, both long term and short term effect must be considered. While achieving harmony of short term and long term is said to be a sign of a high skill (e.g. enjoying pleasant being in the here and now, and at the same time making progress towards Nirvana), sometimes the two cannot be reconciled at which point long term becomes more important than short term. All this follows from Buddha's explanations in Pali Canon.

Also, both personal as well as global result must be considered. Again, harmonizing the two is a sign of class, but in case of conflict -- Mahayana folks close your ears -- personal result wins. Buddha is very explicit about this. It does not mean it is Ok to achieve personal bliss at the expense of the world, vampire style. It just means, if oxygen masks jump out, first put on yours, and then help your neighbor. This is my understanding of course, but I swear I did not make this up.

There is one distinction that Buddha usually drew and that is between the good that leads to worldly happiness vs. good that leads to Liberation. The two match pretty closely in most basic scenarios (like adhering to five precepts in day to day behavior, which both helps social harmony AND involves taming one's mind), but may diverge on more advanced stages:

... Now, I say that right view is twofold. There is right view that has cankers, that is on the side of merit, that ripens unto cleaving to new birth. There is right view that is ariyan, cankerless, super-mundane, a component of the Way...

This is how it basically is... What else... To answer your question directly, if there is anything like "sin" in Buddhism, it would only be so because of its correlation with bad effect, getting one further from Nirvana and closer to dukkha. Buddhism is a very practical framework, and because it grounds itself in first-hand experience it's kinda hard to to argue with.

For a more professional take on the topic, check out this piece by Bhikkhu Thich Nhat-Tu. There is also tons of research on this topic in buddhological academia, but I would be very careful with putting my trust in research that does not come from a practicing Buddhist.

To conclude on a practical note, in case you have doubts as to whether it is in our power to develop kusala and abandon akusala here is some encouragement from The Man Himself:

"Abandon what is unskillful (akusala), monks. It is possible to abandon what is unskillful. If it were not possible to abandon what is unskillful, I would not say to you, 'Abandon what is unskillful.' But because it is possible to abandon what is unskillful, I say to you, 'Abandon what is unskillful.' If this abandoning of what is unskillful were conducive to harm and pain, I would not say to you, 'Abandon what is unskillful.' But because this abandoning of what is unskillful is conducive to benefit and pleasure, I say to you, 'Abandon what is unskillful.'

"Develop what is skillful (kusala), monks. It is possible to develop what is skillful. If it were not possible to develop what is skillful, I would not say to you, 'Develop what is skillful.' But because it is possible to develop what is skillful, I say to you, 'Develop what is skillful.' If this development of what is skillful were conducive to harm and pain, I would not say to you, 'Develop what is skillful.' But because this development of what is skillful is conducive to benefit and pleasure, I say to you, 'Develop what is skillful.'" -- Kusala Sutta AN 2.19

......
¹not really, as "everything is right" would be the god's realm. Nirvana achieves "everything is right" in a funny way, by transcending the axis of psychologically agreeable/disagreeable, which makes everything right even when it's wrong, but that goes beyond our present discussion. For practical purposes let's pretend Nirvana is a simple "everything is right".

Normally, the word does not have any moral sense as some suttas indicate:

The Blessed One said: "Suppose an elephant hunter were to enter an elephant forest and were to see there a large elephant footprint, long in extent and broad in width. A skilled (kusalo) elephant hunter would not yet come to the conclusion, 'What a big bull elephant!' Why is that? Because in an elephant forest there are dwarf female elephants with big feet. The footprint might be one of theirs.

"So he follows along and sees in the elephant forest a large elephant footprint, long in extent and broad in width, and some scratch marks high up. A skilled (kusalo) elephant hunter would not yet come to the conclusion, 'What a big bull elephant!' Why is that? Because in an elephant forest there are tall female elephants with prominent teeth & big feet. The footprint might be one of theirs.

"So he follows along and sees in the elephant forest a large elephant footprint, long in extent and broad in width, with some scratch marks and tusk slashes high up. A skilled (kusalo) elephant hunter would not yet come to the conclusion, 'What a big bull elephant!' Why is that? Because in an elephant forest there are tall female elephants with tusks & big feet. The footprint might be one of theirs.
-MN 27, The Shorter Discourse on the Simile of the Elephant Footprint (Cūḷahatthipadopama-suttaṃ)

In that case, prince, I will ask you a counter-question. Answer as you see fit. What do you think: are you skilled (kusalo) in the parts of a chariot?"

"Yes, lord. I am skilled (kusalo) in the parts of a chariot."

"And what do you think: When people come & ask you, 'What is the name of this part of the chariot?' does this line of reasoning appear to your awareness beforehand — 'If those who approach me ask this, I — thus asked — will answer in this way' — or do you come up with the answer on the spot?"

"Lord, I am renowned for being skilled (kusalo) in the parts of a chariot. All the parts of a chariot are well-known to me. I come up with the answer on the spot."
-MN 58, Discourse to Prince Abhaya (Abhayarājakumāra-suttaṃ)

But it also acquires a moral sense because it is also applied in moral contexts. The next excerpt is an apt example of this:

"Now, venerable Ananda, what kind of bodily\verbal\mental behaviour is uncensured by wise recluses and brahmins?"

"Any bodily\verbal\mental behaviour that is wholesome (kusalo), great king."

"Now, venerable Ananda, what kind of bodily\verbal\mental behaviour is wholesome (kusalo)?"

"Any bodily\verbal\mental behaviour that is blameless, great king."

"Now, venerable Ananda, what kind of bodily\verbal\mental behaviour is blameless? "

"Any bodily\verbal\mental behaviour that does not bring affliction, great king."

"Now, venerable Ananda, what kind of bodily\verbal\mental behaviour does not bring affliction?"

"Any bodily\verbal\mental behaviour that has pleasant results, great king."

"Now, venerable Ananda, what kind of bodily\verbal\mental behaviour has pleasant results?"

"Any bodily\verbal\mental behaviour, great king, that does not lead to one's own affliction, or to the affliction of others, or to the affliction of both, and on account of which unwholesome states (akusalā dhammā) diminish and wholesome states (kusalā dhammā) inscrease. Such bodily\verbal\mental behaviour is uncensured by wise recluses and brahmins, great king.

"Now, venerable Ananda, does the Blessed One praise only the undertaking of all wholesome states (kusalānaṃ dhammānaṃ)?"

"The Tathagata, great king, has abandoned all unwholesome states (akusaladhamma) and possesses wholesome states (kusaladhamma)."

-MN 88, Discourse about a Cloak (Bāhitika-suttaṃ)
Note: I would change the word 'states' for 'behaviors' or 'conduct'.

I hope this answer is enough to meet the question's requirement:

For this reason, I am wondering if anyone can provide more information about the Pāli term, the connotations it had, and the subtleties of its meaning in the context of Buddhist teachings (generally or in specific contexts, i.e., specific suttas).

If not, let me know. I can find some more references if you need.