And yes, I understand the sheer power of anti-western sentiment at the Roman capital at this time, but the idea of any TL involving Maria and Renier is that she will be the focal point of the regime and the "public face" to the mob of Constantinople.
Manuel would likely have forced his senior commanders to swear oaths of loyalty to his heirs as well, not that those would prevent rebellion, but for commanders such as the Megas Doux Andronikos Kontestephanos, this might have prevented his helping Andronikos I capture the capital.
In the end, I am wondering if a "Tamar of Georgia" like situation is possible for Rhomania. Please tell me if this is a ridiculous idea though!