If you want to make the unplayable factions playable, perhaps you need to do some extra modding to descr_strat.txt file. Currently some of the factions have no starting leaders or settlements and instead have something like this:
faction romano_british, balanced caesar
dead_until_resurrected.

That suggests that merely adding those factions to the playable section of the file will do nothing but cause troubles. Some modifying of starting settlement ownership and creating a few characters for the factions awaiting 'resurrection' might make them playable from the start, but then again, might not. I'm sure someone will give it a go if they haven't already.