I tested this and based on my testing it’s (3). The AirPods microphone actually gets (slightly) better in quality if you don’t play something through the speakers while using it. Even then the 3.5mm ones were better tho

4) The Bluetooth standard is just kinda bad. Technically there should be enough bandwidth for better quality audio, but the existing profiles like headset mode drop audio quality down to 8-16 kHz sampling rate in mono, not just for mic but for audio you're listening to as well. It's a huge flaw and it's been bad for decades with seemingly no improvement at all (at least certainly not in a way that could be widespread and commonly used).

It's so bad you'd almost wish for a brand new wireless connectivity or wireless audio standard, or even resort to some proprietary 2.4 GHz nonsense, because it's genuinely so horrid. You could have the best most expensive headphones in the world, but because of Bluetooth and its ancient profiles and mic support it's gonna have baseline absolute garbage mic quality no matter what.