In case anyone else was curious, I asked Keith on Discord and this is what he said
On A, if by "Other AIs" you simply mean "Nemeses" , then that's fine. I'm assuming you mean ones that are "free" from the beginning of the game, given your later items
On B, yes, you can define new units/structures (with hover text and all that), and you can mod mapgen to put those structures wherever you like
On C, yes, you could have units cause a taunt/message when spawning; and the tutorial engine will certainly have hooks for messages at the start of the game (among other times); though any other specific times that you have in mind would be good to mention
On D, yes, you can have one unit spawn another either as part of "firing" a particular gun/system, or as part of dying (by having a fires-on-death system)
On E, yes, there's no special restriction on nemeses targeting one another or basing their logic on one another's position/etc; that would be done by writing the custom nemesis logic
On F, yes, there's always something :smiley: