First, Mullvad's infrastructure has been independently audited.

Mullvad integrity has also tested as proven by a legal case where they were subject to a search warrant when someone was trying to claim copyright infringement.

As far as I can tell, Obscura has not had anywhere near the same scrutiny.

Second, obscura is the first hop is it not ?

Therefore it may well "only" relay the traffic to the exit node but it is still a relay and hence open to SIGINT analysis by the US.

I would have thought therefore using Mullvad's built-in multi-hop mode on their audited platform would be the wiser decision ?

Or Tor if you insist on multi-party ?