What a shame to have opened such a can of worms unnecessarily. I'd be real tempted to have your "surveyor" back out. I might even turn the hose on him. Hose him down real good and suggest he use his moisture meter to determine if he was wet or not. Not helpful. But satisfying.
Yes. 3 inches on a 1/4 thick inch laminate is the recommended bevel. But more won't hurt. Make sure the bevel surface is flat and not made concave by the edge of your grinder.
One mistake to avoid; Maybe you already know this, but when it comes time to start laying in your laminations, put the biggest one (and oversized) in first. Then the successively smaller and smaller diameter layers on top of it until the hole is filled. Then sand down to level and final fill and fair.
This may seem counterintuitive. But if you do it in the opposite order you will sever all the fibers of the largest cloth layer that you put on when you sand it down to level and fair. So the biggest top layer will not actually be bonded to the hull
. Only to the smaller patches. I believe the Gudgeon brothers developed this technique. Lots of good info from them online.
Another technique you may want to consider is bonding the original piece that you cut out back into the hole. You didn't mention the size of the hole you cut. But this can save you considerable effort in fairing and may provide an even stiffer patch. Just cut the same 3 inch bevel on the removed piece that you cut on the hull
You'll also want to put some thought into what resin to use. Lot's of conflicting info about resins and their relative bond strength and porosity out there, too. I'd want to know what resin was used by the builder
. The last thing you want to do is create a porosity or compatibility problem where none existed before.
I would go with a 6 inch variable speed DA sander and maybe 36 grit discs. And go slowly. Any kind of high speed grinder works too fast for me to control such fine work. And the heat generated by high speed grinding can compromise the strength of the resin and fibers you are bonding to.
As far as uniformity is concerned, you don't need surgical precision or micrometer conformity here. Pretty good is good enough as long as you err on the plus or wider side. And a coarser grit will give the surface more tooth for a better bond.
And you do know to wipe down with wax and grease remover before sanding
as well as after? Otherwise you grind contaminants into the surface. Lot's of people make that mistake. And grind mold
release wax into the surface.
Structurally, as a bonded foam core sandwich, your hull is plenty stiff. I would be more worried about porosity than flexing. And modern foam cores and resins don't automatically delaminate the instant they get a little damp.
Personally, I think a hard plastic mallet and your ear is a better way to check a hull than a meter.
Don't obsess over it. If you screw it up, you can fix it. Over and over again as necessary!