Phone calls use a different Bluetooth profile, so that's largely irrelevant.

1 megabit (did you mean megabyte?) of memory is not worth noting. It could be integrated into the IC that contains the entire system for headphones quite easily. That system already has memory in there anyway.

1 Megabyte and your average BT audio device has less memory than you think

Standards have not been made considering today's availability of memory and resources

I'm not sure how much memory you think I assume is present. My point is that it would be a possible and simple extension, not that it is already present.

There is currently exactly as much memory as needed. That does not make it unrealistic to extend it with a buffer. Cheap SBC-only devices certainly won't do it as those are solely about cost optimization, but the beefier, pricier AAC/Apt-X multi-device capable bluetooth audio sink solutions that sell for premiums could easily incorporate a bit of extra memory. 1Mb (megabit) would give a few seconds of buffering, which would be sufficient. It'll take a bit more silicon, but nothing absurd. Hell, it could be external memory over SPI, so integrators could select buffering capability themselves, without significant cost/silicon area increase of the bt chipset

However, to actually utilize buffering, you'd need a higher bandwidth link than is required for live playback to catch up after connectivity dips. Buffering by itself only adds latency. The question is then whether you'd want to use your link bw for resilience of low quality audio, or instant high quality audio...

Usually far less than 1 MB. It's about cost, footprint and power consumption.

What profile is used for phone calls depend on configuration of the phone and the headset.

Last week I accidentally found out that I can use two phones at once with integrated HF in my car's radio as long as they use different BT profile (somewhat unsurprisingly, while trying to fix completely unusable sound quality over BT)

That's so you can have a music device and calling device connected simultaneously. Both connections can be consumed by one device if you want both music and call functionality.

I don't think your phone will try to use A2DP for a call.