For what ever reason I'm would say you are only getting a signal via the antenna and not the cable.
The initial problem started AFTER the wall mount had been replaced. I would think that everything from the wall connection and beyond should be ok, unless you experienced a power issue at the time of switching.
I would revisit the tv area where the brackets are. Something has change here. Maybe get a friend to have a look. I know myself during troubleshooting after awhile one can become blind to the obvious.
You have got me quite curious on your delema here. To bad I couldnt be there in person, it would make this easier.
Please let us know when you get this resolved.
PS i am sending this from my phone. Bound to be tpyos.
07- Classic 2500 LT CC 4x4 SB Duramax/Ally
13-320RLT Crusader TE
Reese 16K Kwik Slide/Prodigy brake contoller