I favour option #2, if it's found to be widely compatible with existing clients.
To address that point, and add "For instance..."
We may mention this use of User-Agent sniffing to determine client capabilities in passing in the draft.
Good idea.
It seems not profile matter, but protocol matter, so I did not made change.
Addressed Rob's comment. https://mailarchive.ietf.org/arch/msg/spasm/OWDDnWVnSrCq27n5qSQk9nBiS9c/
Addressed Russ's comment https://mailarchive.ietf.org/arch/msg/spasm/qtTbW5DPDMbYqDJ5h8CGMxn1QMM/