Welcome to the wonderful world of the Seawind. I can not attest to the Motor mount fix as I have not needed it and it looks just fine the last time I was in there. I will say that on Thursday the wood mount in the fuselage just let go. It was the whole thing. I have had many flights before this happened but I have taken my wing off and back on several times. I just Gorilla glued it and taped it down for pressure so the GG can set up and I left it as is and have flown all weekend like that with no troubles.
I do not think you are hurting anything with the larger motor but I will say it does not need it. You weakest spot in your setup so far is the esc. First of all it can't even handle the factory suggested motor and will be a waste of aircraft on the larger one. The GP esc's have giving me nothing but trouble from day one and I would recommend investing in a Castle Creation Phoenix 25 or if you don't wish to go that spendy you can use a BP Hobbies 30 amp esc. I know people that swear by them and the price is just right.
Here is the link for the BP esc.
The Futaba servos are about as good as it gets for this plane. They are small in size and have excellent torque. I will be outfitting my PBY with them. My Seawind has Eflights s75 servos which are a little larger and weaker but they work just fine so you look to be in good shape as long as you get rid of that ESC.
Hope I have helped.