Ok, so I tried out the KBS (aka rust stops here) and I was happy. They claim that the better you prep, the better the result. Makes sense. I bought their frame kit for $62 and it came with cleaner, rust neutralizer/etcher, and a quart of paint available in ten colors.
I cleaned it well with wire wheels in the light rust areas and even a light grinding in the heavier areas before wire wheel. The cleaner still lifted more out, especially if you wet sand with 320 as they recommend. The etcher is just a mild phosphoric acid but it worked well and is part of the kit.
The paint is thin enough to spray with a gun but I chose to brush since its inside a hidden area. The brushes they provide suck so use something better. You can clean them but I didn't care to try since the product is impervious to solvents once dry. They recommend two coats about 3 hours apart. The brush strokes flowed out really nice.
They claim you can top coat it. I may do the entire engine bay and front wheel well area since it's pretty rusty all over. If I do, I'm going to use black in the wheel wells and leave it be and use white in the bay covered by a good epoxy primer sealer then top coat it.
The stuff is hard as nails once dry
340ChallConvert....I think you are going to be forced to section in a new piece in the floor pan where the holes are. Once it gets holes like that, the metal is rice paper thin. I have tried weld filling those on things I couldn't replace and it's a major effort. The welder just blows holes in the surrounding metal. You can put little patches over the holes but it's really not a proper fix and looks terrible. Those floor repair sections are not pricey and A decent shop should be able to butt weld in new metal at a fair price. It's not a big job for them.
On the other areas without holes, I think you're on the right track. I've never tried por15 so I don't know much about it. Go to the KBS website and watch their how to videos.
On the bottom side...what I do to make it easier is put all four corners of the car on super heavy duty jack stands (obviously under solid front frame rail and rear housing points for safety) the heavy duty stands are cheap and go really tall. I put my projects in the center of the garage and high enough that I can work comfortably on my back.
I would do it once and do it right by removing the transmission, driveshaft, shifter hardware, and e brake cable. The best thing I found for undercoating is the gasket scrapers from craftsman. Use a file to keep them sharp, some brute force and you can strip 90% of that nastiness off in a couple hours.
According to KBS, the best way to really get the paint to bond to the rust is to get all the old paint, undercoat, rust off. The first pic you showed after wire brushing is what they recommend for good prep.