Bringing in more birds may only add to your problems....stress of integration, not to mention the biological risks.
Consider biological/medical quarantine:
BYC Medical Quarantine Article
Poultry Biosecurity
BYC 'medical quarantine' search
Read up on integration..... BYC advanced...
Absolutely.
Most layer feed is pretty low in protein(~16%) adding all that other stuff is diluting the minimal protein in the feed.
Stop all treats, unless high protein treats....might not get you egg right now, but will be good for birds in the long run.
I like to feed a flock...