I had BIOS version 1 and the i9-14900k was recognised without any problems. I only updated the BIOS after installing Windows.

Yes

yes, BUT you have to update the BIOS (7.01) and as there is no longer a flashback button on the motherboard, it's impossible to do this without a previous-generation processor. Personally, I was sold a 13th-generation processor at the Digitec counter, which I was able to return for 10% of its price (CHF 24.-).

I have an I9 14th on it and everything's fine. There's no point in putting 4 RAM 2 Sont more efficient, 2x 32GB are more than enough, after all it depends on what you're looking for.

Yes, there are models that are compatible with 6400Mhz. For example these: G.Skill Trident Z5 (2 x 16GB, 6400 MHz, DDR5-RAM, DIMM) All references can be found on the manufacturer's QVL list: https://www.datocms-assets.com/34299/1672822363-nzxt-n7-z790-memory-qvl.pdf