yes, albeit in a rather limited fashion - this chipset is not designed for this use case, range, speed and maximum number of connecting clients are limited (quite severely so), as are the allowed interface combinations…
no, hard no for brcmfmac.
I wouldn't want to rely on it for that purpose, but neither on your 3. option.
yes.
While this sounds like splitting hairs, there is an important distinction, it's 2.4 GHz XOR 5 GHz, not both simultaneously - in terms of stability and the rest, the same answer from 2. applies.
For wireless AP or repeater usage, a purpose built -even cheap and low-end- plastic wireless AP (starting with e.g. mt7921a (mt7915 wifi or better) or ath79 (ar9280 wifi or better)) will run circles around anything RPi based.