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.
But theymos obviously doesn't think so, so this argument is a mute point in someone's merit source application thread.
As far as I know icopress's merit sharing history has been exemplary so far and there is no particular reason why he shouldn't be a merit source, and I think it is unfair to judge someone based on some speculations about what could happen. Does that mean no one should be a merit source due to potential misuse?
it means no campaign manager should be a source.