For things like the pumpout and water fill and the $700 worth of required electrical connections, I did nothing. The electrical had their own gaskets and I simply cut holes into regulat core.
For deck fills, if you bed them in butyl, they should be fine over regular core as long as you don't go crazy on torque.
The m200 is fine. I prefer aquaplas for screw holding.
I planned all cleat locations on the Skoota, but botched on the bow eye. The cleats have hardfills and all will have backers. I read somewhere each cleat should be backed to support a minimum of half the boat, and I did some calcs and decided on 24 inch square backing minimums which takes my 10,000 pound design to 416 pounds per inch (too high, but what I am using). I have plywood decks in place for two of them, hindsight would have been M200 as one locker filled with water last year and the ply swelled post finish and cause some issues and broke the laminate in one spot. That locker simply did not have a drain installed and we got hammered with rains last August.
Here is the electrical and water. The stuff is on the foam just above a plywood core which we did because the ply gets narrow at the beam..
There is extra glass here and I got lucky because only on raw core, but because on extra glass (this is a chine), nothing is needed. The vent hole is bedded on a big black rubber washer.
If you end up over 4-5# core, and you are worried; decore 2-3x the bolt sizes and refill with milled fiber and epoxy and cabosil and bed in butyl. Limit is about 3/4" hole or it'll heat up and crack on you.
You can always, as a matter of last resort, build an M200 block and make it proud of the hull. I am doing this option for a camera mounting base on the cabin roof. Not done yet.
Rubrails are simply screwed onto raw core every 6". I am nervous about them failing and I bolted them on where I could access behind and bedded/sealed in 4000//4200/5200.
Those hull windows, only bedded in butyl. I did not even decore the raw edge. If I see any issues, I will take them out and chop out the edges and fill with thixo. Top windows are bedded with sase neoprene foam, no core modification, in corecell either pvc or m80. Under the middle window up/escape hatch, there is a diesel heating system fill and vent, also raw core, bedded in butyl, just screws in core where there was extra glass. No access behind it to do much..