"Lack of" in what sense? Being dual-standby instead of dual-active? No, that's up to the manufacturer to implement in hardware. Android has supported it for years.
That sounds like a specific phone issue. I had a dual SIM phone before, a ZTE Axon 7, and had two separate SIMs in it. Never encountered such problem.
No, in the phone settings, you designate one SIM as primary number. Android auto simply use that number to dial out.
Good luck with that. Android Auto is all about less driver distraction. Have to pop up a SIM selection dialog and require driver to pick one for each outgoing call is a huge distraction and I won't expect Google will ever do that.That won't work for me. I want it like how it is when I'm not in AA mode where the phone prompts me to which SIM I want to dial from.