I ordered basically all of my stuff through D&D as well, and they are a great resource for advice and parts. Yes, the original engines used cast pistons and they’ll be fine with a working fluid injection. The Jet Hot coating was just an added precaution. And yes, absolutely, using the thicker head gasket will significantly lower the compression. I did the same thing (Jim Noel can give a dissertation on that subject!)
I guess my big question given your situation would be whether it was the bearings themselves or the actual cam bore that was the issue on your original installation. Like I said, sometimes the bearings can be mismatched or inconsistent in size and that can be the problem. The only way to know that is to measure the cam bearings with a micrometer prior to installation to ensure that every one is the same size or within the spec. I would guess a machine shop would do this, so then it’s down to the cam bore. If it is the bore that’s the issue, machining the cam bore is the solution. As it sounds like you’re not doing this work yourself, you have to trust the professional that’s telling you what the problem is. I definitely put some extra funds in along the way, as it pretty much is with any engine build. It always costs more than you thought it would!