Whoah... you couldn't find the attacking monks? The trick was to find the monk that was in convert animation. Big MUs, I thought, would be like AoK heros: try to convert, and an annoying message would appear at the foot of your viewing window that tells you "you can't do that," then lingers there for about five minutes.
In AoK, most of your list issues were resolved: converted builder guys could build only what your builders could; a converted enemy archer, or any unit, would keep techs it already knew, but could not gain any more; converted buildings could only train what your equivilent buildings could, given there are any: i.e.: Playing as the Mayans, I convert a Celtic stable. The Mayans can't build stables, so I can't train any units there. I convert an archery range, I can train archers and skirmishers, but not cav archers, which the Mayans can't get. It's all in the AoK manual.
And losing through priests alone: It was probably possible in the nutty AoE (Eggy priests with 16 range!), but AoK fixed that very well. I've never seen a 20-man army lose to conversions. I only use it in city defense where the unit would die anyway, or against an AI-style single-unit counterattack. Sorry about your testers, but wouldn't "banish" also discourage them from building MUs? And, like in your (really funny) other thread, the guys should have enough defenses to protect them anyway... Conversion wouldn't have been the only way to get MUs, after all. This fix is sort of delaying the inevitable. Plus, countering priests is easy, so long as you have ranged units.
So you kept the code in but took out the feature? Are you hiding something? (Of course, you guys wouldn't ADMIT it, but it's worth a shot!)
Four months isn't enough time to manifest some code that's already there? Even if you use MS Office to do it, you still could fix its "suggestions" by September. I'm sure that this has gone on far enough, so if you're still reading, 1) you have too much free time, and 2) I can solve your priest problems without major fundamental changes in the game.
Edit the priest properties and set "range" to 4(-ish).