1) I hear you on the fresh air blower, would suggest you prioritize it below the heat and AC for now.
2) I've owned my Carrera for around 20 years and have replaced my footwell blowers several times -- they use motors/bushings that deteriorate/fail over time -- not clear whether you've powered the footwell blowers up independently to confirm that they both work -- blown footwell blower fuses would point to seized footwell blower motors (or very squeaky footwell blower motors that are creating too much resistance and blowing fuses) -- believe that a failed footwell blower OR a failed trunk blower motor will take the entire circuit offline, which could explain why your replacement relays didn't work.
3) AC -- is a whole other can of worms -- shortish version -- even when new, our cars' AC was marginal at best. Further, the hoses carrying the freon are all gas-permeable, allowing the freon to slowly leak out (and then blow warm air) -- this caused me to abandon my AC when I moved from the east coast to Denver many years ago. I'm now in the process of getting my AC operating again, but this is requiring me to a) replace all the hoses with new, non-permeable hoses, b) replace the compressor (my trunk deck had a oil stain just above the compressor belt, indicating the compressor's seal had worn out), and c) replace virtually every other part save the front and deckled condensers -- My mechanic is using parts from
www.griffiths.com for this (and will be converting my car from generally NLA r-12 freon to r-134a) that should make my AC slightly more effective than new. Others on this board have a LOT of good things to say about Griffiths (and forum user Keuhl is also the owner (?) of Griffiths) and Griffiths has options available (rear wheel well condensers, etc.) that can significantly increase our cars' ac cooling capacity -- There's also another 911-specific option available from a forum member/vendor out of the UK that totally ditches the engine-driven compressor and is electrically driven -- either way, point here is that unless your AC system's already been recently upgraded and gone through, it likely will need professional help (and $$$) to get working/effective.
So, at the end of the day, you're in a situation similar to mine a few years back -- I was able to get/keep my heat working (which was crucial to cold weather driving/defrosting windshield/etc.) but used "nature's AC" (opened a window/popped of targa top) on hot days, but with some $/time I'm finally tackling my AC