1st of all, I am not buying that business about the wood will have enough resin on it for the seacast to stick to it. I don't believe that the surface of the side of the wood you will be trying to adhere to will be resin coated, unless you do it. Her assurance sounds like someone just trying to convince you that their product is easy to use. It either adheres to wood or it doesn't. I would try very hard to get as much of that wood out of ther and then I would lay down a coat of resin before appling the seacast. Or I would use a different product that does adhere well to wood.

If your scuppers are below the waterline now, then they will be even more below the waterline with a bracket mount, all other things being equal, because the weight will be farther back from the center of gravity.
I don't know if your bracket will be providing flotation or above the waterline adding weight also.

Do you need a full transom? Will you be boating in "big" water. Drifting in tidal inlets, for example.

In another discussion about the stern weight issue, I posted that the anchor roller/platform system that I installed (about 70 lbs total) very nicely balanced out my kicker motor because the weight was so far forward.

Whatever time you think it is gonna take, you can count on it taking longer. It always does. ;D