Oh! An actual Sisters of Battle player! Great!
Also, Warhammerman is a woman
? Who knew?
Anyway, I like the idea of a dominion squad in an immolator to get those meltaguns up close and personal ASAP. I'm also happy to drop the penitent engine - it's an awesome model, but I agree that walkers aren't what they were... if they ever were much of anything.
Regarding who gets the simulacrum, the reason I put it on the battle sisters squads is that they are the ones whose Act of Faith is actually useful to this army (reroll 1s to hit? Instantly regroup? Yes please!). The canoness and her command squad, on the other hand, have Acts of Faith that give them Relentless and Moves Through Cover, both of them useless to a mechanized unit.
Also, while I've got your attention - oh player of SoBs - what do you think about dropping the command squad to pay for the dominion squad and putting the canoness with them in an immolator? It saves me a few points for upgrades (actually, enough points for a freaking Baal Predator, so all that rhino chassis distraction actually does me some good!), and I'm not sure what the command squad really adds.