Actually, it was imprisoned by Omega, not the Garleans, and we don't know all the effects that field had on Shinryu. On top of that, all other Primals need worshipers to maintain themselves in addition to crystals, hence why Primals 'temper' people. Shinryu's initial "worshipers" all died before he was manifest and he didn't have time to put anyone under his sway.
For Zenos, I did say that I can't speak to how we exploited his weakness to overcome him, but we had already shown signs during the second encounter that we were starting to rise up to his level. Once more, there's a LOT we don't know about the Echo and how it actually works at this point and that really becomes our "plot armor" in the end.