for steam version, you need to use the Steam beta program (which in this case just means that the devs stored the old patch for people who wanted it) and choose the 3.19 one. It should be somewhere in the Properties options of your game.
For the rest :
1) Scions do not have any particular issue to my knowledge in the svn build
2)D'tesh is the worst offender but has been disabled by default, others i don't really know ( sheaim tend to do poorly in my games, but i'm working on it)
3)All modules in the beta version are compatible, others can be made compatible easily if needed. (I can do some if enough people want to)
4)never seen such an incompetent AI, teching, building, and land-improving are globally alright ( not great, but alright)
5)No plans to do it, but no plans not to do it either, i've never really looked into mapping so i'm not sure how much work it would be. If it can be done easily, i'll do it.