Plan 100% successful. In order to gain the legitimacy necessary to create the Roman Republic, I found some 50-year-old Cathar in my realm and took her as concubine, AKA publicist.
She taught me the secret voodoo words, and it was done:
http://steamcommunity.com/sharedfiles/filedetails/?id=479675347Here's another image just to show off Scotland:
http://steamcommunity.com/sharedfiles/filedetails/?id=479676285Carthage says hello, bitchez!
Anyway now I have a very important decision. Thanks to donating several thousand to the Spirit Guardians the day before my "conversion", I have plenty of piety to go right back to West African. If it's anything like the Byzantine Empire, the game only cares about your religion when you form the title in the first place. So I can switch back, and my massively powerful barely managed prince-mayors won't be inclined to murder me. I go on to SPQR in the name of The Spirits and The Ancestors, who got me this far.
Or... I stay Cathar. Which seems painfully ironic for a trade republic, a literal patriarchy so locked into agnatic that mods can't fix it (much like Itqa). Buuut, my retinue is about as large as all my vassals put together. They've been running popular independence factions almost since I left Africa, but never fired (despite me squandering hundreds of thousands of my retinue in my haste). So I mean... I could just revoke their kingdoms. My retinues are costing about 2.5k gold at full reinforcement... I have 266k. Even with 1500 family dues, I could go north korea for about 60 years! And I don't even need too because of seniority - ever 5-15 years I "die" and all is forgiven. Which brings in even MORE gold from all those family dues coming back to me.
And once I revoke the kingdoms, I hand them back out as feudal, Cathar viceroyalties. I can't force them to go absolute cognatic, but hopefully some will.
... I was going to ask whether I should go Cathar or West African, but I think I talked myself into Cathar. I might not finish before the save-breaking patch - but screwit, what do I care for some S.P.Q.R achievement? I've come too far to do the sane thing