Man.... relocating the main stack so you can put in some shelves??

Code compliance for air-admittance valves vary by jurisdiction, though, they are usually used for one fixture, not the main stack.

I would find available space elsewhere and leave the stack alone, imo.

I know, it's a little silly to do but when you need storage for woman stuff, towels, etc - a man has to find a way? A bolt on cabinet wouldn't add enough space on it's own given the size of the bathroom. I am tearing it down to studs and redoing nearly everything anyway so a couple of extra hours for valuable storage makes sense, everyone loves built-ins.

it wouldn't be too be bad, I would just be installing a C shaped piece of pipe with a fernaco elbow towards the bottom. Scrapping the copper pipe might even offset the expense of the 'upgrade'. Do I need to keep this 2" or can I reduce the size? is there any flexible hose that I could install instead of rigid pipe and still be code complainant?

I know I can use the admittance valve based on code, just no idea if the air intake would be enough to support 2 toilets flushing at the same time, both the shower and tub running.

the plumbing in this house is a little odd to begin with though; there's another main stank on the opposite side of the house attached to a utility sink, and washing machine; the kitchen has none, just a counter mounted air-gap unit;