I want to trust manager x.
Then check his trust feedback. 
First I can’t see his long term merit history.
Sure you can. Then again, I really doubt that an average business looking to advertise here cares about manager's merit history or even knows what merit is. So if you really want to allow any manager to be a merit source you need to have a list of every merit they ever gave.
There is already at least one active manager who is also a merit source. and whom ever he is or she is they should not be a merit source.