Uh what? Romuva is probably the easiest religion to reform. I've seen the AI reform it more than any other religion.
Also you did do this, by your actions against the Catholics and other religions. Since you've heavily hurt the ERE enemies and allowed them to spread while leaving them untouched, you have stacked the deck in their favour.
I started this campaign around 20 hours into the game, I was still learning all the mechanics. This was my first serious game. I didn't play in Ireland because I watched so many tutorials before I got the game.
And your analysis on the reasons the Byzantines become huge is correct. I smashed the Catholics and the Muslims, but I'm a bit of a Byzantine fanboy