You will not be able to build Manziel's accuracy ratings up enough to make him a legit QB 1.
For starters, his SAC rating is abysmal, so we'd essentially have to start there. Most legit QB 1's have SAC's in the high 80's - mid 90's. Manziel will never be afforded that opportunity because of his Slow dev trait. Accuracy ratings for QB's take a lot of XP to build, so it would probably take you two full year's worth of XP, plus using a slot in gameplanning to train him weekly, just to bump his SAC to a respectable 85 and up... but with regression, there's no telling what could happen at his age. If we were talking about a Superstar like Newton, this would be a completely different story.
Play Rivers til you get your real future at QB. Ask yourself, would a real NFL team trade Rivers as their starter for a
safety to fill a need, and bank on Manziel of all people? No. If anything, they would do the exact opposite if given the opportunity. Without knowing a single other thing about your roster, I can guarantee you that your roster is better off with Rivers as your starter than Manziel + TJ Ward.
If you want to develop a QB, dump Manziel and draft a QB with Rivers as your QB 1. Never, EVER, trade your QB 1 like Rivers hoping a guy like Manziel can fill the gap, because he simply can't and won't ever be able to.