The bolts I received from GM will tap themselves. Just go slow, thread the bolts first without the plate on there. You are correct though, don't over tighten them or they will strip out easier than a normal bolt and nut that size. This applies to all the bolts you receive if you order all 3 plates from GM.
The doubling up on the transfer plate seems overkill to me. These are not made for rock crawling if you want to support the weight of the truck on the transfer skid plate you better get a custom one made with much more bracing.
My doubling up by spooning the shields would at least make it a little stronger and perhaps give you more warning time and require more force to biff the TC. At the time this was originally posted the plates were pretty cheap.
No, these 2015-2020s are not good at rock crawling but can at least be made to approach or equal the old GMT400's handicapped abilities. I made it a point to buy one with a 'real' 2-speed TC
I took mine out last summer and it did pretty well creeping over some challenging (for it) rocks. You just have to go glacially slow. One of the chief problems was downward body lunge. All the more reason to have the skid plates. Yes, they do need to support or nearly support the full body weight.
I plan to have one custom made as there are none on the market I'm aware of.
As to the stock skid plate install; I advise tapping the holes, using silver anti-seize compound and just snug them up (don't over tighten)
General Note: Yeah ... IFS sucks and it will always suck, but plenty of people make do with it and unless you want a truck that tops out at about 87mph you really can't buy a new vehicle with a solid front axle. I have to drive 800 miles to reach the places I like to go, so compromises needed to be made, otherwise I would have done a rotisserie resto on my '90 Blazer.