StickyIcky, we lost our KiKi to the tainted food last year so Im probably paranoid but at the same time we are feeding commercial food to our new girl so I might not be paranoid enough, lol  I think all you can do is do the best you can.  I think it helped me to read thru alot of the info here at Itchmo on people's experiences with food section and just do the best you can.  I think we have all learned that sometimes its what the pet does best on is the food to go with as each pet is different.  Im sorry, I dont know anything about Merrick so I cant help you there.  Ive been down the road you are on, feeding one thing and then hear some not so good stuff about it and then change the food, and so on.  Im only posting to give you hope that at some point you will find something you feel comfortable enough to stick with, if it can happen with me, it can happen with you too.  Although thats not to say I wont switch what Im feeding next week if something comes out about the food were on......or then again maybe I wont or then again.........oh well I should just stop while Im ahead.   
Sorry Im not much help.