[Gregstrom]

1) Reincarnation is as far as I know random - you can't specifically trigger it.
2) See 1.
3) I vaguely recall JK once saying that to have a chance to reincarnate as a higher form they need to have lived virtuous lives - no fighting, for example. I don't know the exact mechanism - maybe lch can say more about.
4) One example of higher form: The unit ascends to a state of nirvana. You don't get them back, and you get a boost to your dominion.
5) See 4. Also, as noted above the lesser forms include other types of recruitable ape.
6) Again as already mentioned, age does not appear to be reset.