What makes you think your wirenut kludge is less "iffy" than using the IDC connectors that are designed for the purpose?

My experience with the piercing type is that the connection is not as secure. I've had to redo a number of them during installation previously, and I'm a pretty capable handyman. To me, they fall in the same category as replacement plugs for zip cord that depend on piercing of the insulation, or back stab receptacles: just not as reliable. In addition, I have two locations where I am going to have cable splices because of the lighting design layout, so I need to use some type of wire nut.

Having said that, what's your opinion on my original proposal?

I think your original proposal is iffy. I recommend marine grade butt connectors. 100% silicon contains acid.

20 pack for $11 max 4-#12

My guess is the little bushing on the bottom of the cap helps keep the silicone in the cap for the waterproof seal.

Edit:

HD has them cheaper and possibly local. 20 pack for $9. About 45 cents each.

Also, I believe there is dielectric silicone grease in there and not silicone caulking. Dielectric grease is used for spark plugs in automotive also.

Perhaps knowing that, you could invest in some of these and if you end up making a modification you could reuse them by adding dielectric grease. They say not for reuse, but most likely because after the first use the grease is not sufficient for a waterproof seal.

They make a can of brush-on electrical waterproof sealer. The home centers stock it.