If a kind and generous Christian died 20 years ago, might their rebirth have resulted in there being a 19 year old person in the human realm now?

To be born as a human, you need to keep to the five precepts well. To be born to a Buddhist family, you need to dedicate your good deeds towards attaining Nibbana. If your friend died 20 years ago, he could be 20 human years old now, wherever he is born, unless he died again.

Rebirth is using the conventional/conditioned truth of suffering, 'I, me, mine', to extrapolate, infer, into the unconditioned truth of the end of suffering, 'not I, not me, not mine'.

Majjhima Nikaya 27 does not allow for inference to grasp ultimate truth, only direct observation. In this respect, if you engage with rebirth, you are already one step away from the Tathagata's message of the cessation of suffering, as opposed to his message on suffering.

The issue is Dhamma is regular, without heights or extremes. Bardo naturally creates the extreme of existence and non-existence, as is therefore philosophically unacceptable. Spontaneous but memoryless continuation of the conditioned - that has NOT reached the unconditioned - is the only option that is not irregular.

That the conditioned randomly (not chaotically, that is still mathematically defined) wobbles around the mathematical approximate of emptiness that is '0', is irregular, so untenable for Dhamma.

Isms don't play any factor in the Newtonianesque law of kamma-vipaka. Intentional action and the result of intentional action, gives no concern for 'isms'.

Wisdom - the ideological space, and concentration - the supramundane path, are considered supreme within Dhamma, but that is different from the morality section.

There is no supremacy within Dhamma with respect to morality. Intentional action to non-harm, non ill-will, will bring benefit whether acted on by a murderer or by a saint.

A good Christian, within Dhamma - subject to accepting 'I' - will be reborn where they stand with respect to their morality.

It's hard to say where he is born (whether in bad places, human world or hevans) by referring to what he did in his normal life. Why I'm saying this is according to the Sotaapanno Sutta: The Sotaapanna ('Stream-winner') (quoted below) only stream-winner (or above) has the guarantee that he will not go to hells. That means only stream winner (or above) will not get unhappy births which are Thirisan (animal's world), Pretha, Asura or Narak. This also doesn't imply that only stream-winners (or above) can get happy births. It's only about the guarantee.

There's a story of a lady called Mallika in buddha's days. She did a big almsgiving (dāna) called asu disi dāna those days. But her immediate re-birth was an unhappy one. Why? it's because she was not a stream-winner. So nothing can be guaranteed. But the good work she did has gone to her karma and she will get those back in one day. (if my memory is correct and she spent only 7-days in unhappy birth and after 7 days she got a happy birth again).

This is why being a stream-entrant/winner is so much in our life. And that's what Buddha has advised to do as the first thing in our life.

"And when, monks, the Ariyan disciple understands as they really are the arising and the passing away, the attractiveness and the danger, and the deliverance from the five groups of clinging, he is called an Ariyan disciple who is a Stream-winner, not liable to states of woe, assured of final enlightenment."

If you attain stream entry in this lifetime, your next birth as a human being is assured. Stream enterants take next birth at most 7 times, as a human being, before they reach ultimate enlightenment.

To reach stream entery you must duplicate, understand, contemplate and realize the complete teachings of Buddha. Then you must make 100 days of concentrated effort of meditation. This process is described in the ancient Taoist scripture, Secret of the Golden Flower. I recommend reading The Secret of Secrets vol. 1&2 for complete understanding of this process.

From the Tibetan Buddhism perspective this is a possible outcome. In philosophy David Lewis calls these possible worlds. It might even be true in the actual world. Which is the world we live in.

The bardo and rebirth would have to happen first of course.

Also human rebirth is considered to be rare, rarer still is human rebirth that is interested in the dharma. But it seems if the worldview is correct, that human rebirths would become more common.

But all that aside, the human realm is still Saṃsāra. Being reborn is rough, relearning everything takes time. Where they were born and to what families will determine what they learn and in what order. I think that covers the first question.

As for the assumption after the first question, kindness and generosity is said to result in wealth, and being in safety respectively.

I do not understand what you mean in your last sentence though. You mention one of the 3 vehicles.

The Buddha taught in many suttas the kind and generous go to heaven.

I think it's difficult to talk about rebirth as if it were a sure thing: for example because kamma comes from a being's previous life, but it also comes from the life before that, etc.

"The [precise working out of the] results of kamma" is said to be unconjecturable.

Actually being able to directly see others' karmic destinations is one of the miraculous powers attributed to the Buddha:

Then, the records tell us, in the first watch of the night he directed his concentrated mind to the recollection of his previous lives. Gradually there unfolded before his inner vision his experiences in many past births, even during many cosmic aeons; in the middle watch of the night he developed the "divine eye" by which he could see beings passing away and taking rebirth in accordance with their karma, their deeds; and in the last watch of the night he penetrated the deepest truths of existence, the most basic laws of reality, and thereby removed from his mind the subtlest veils of ignorance. When dawn broke, the figure sitting beneath the tree was no longer a Bodhisatta, a seeker of enlightenment, but a Buddha, a Perfectly Enlightened One, one who had attained the Deathless in this very life itself.