What concerns me, and is not immediately clear, is how they are getting the initial connection information from Mullvad to your client in the first place. The tunnel might be relayed, but if the initial key exchange was handled by Obscura at any point, then they could potentially decrypt that tunnel breaking E2EE.
I believe other private relays use some sort of private token system to authenticate to the exit nodes without the entry nodes ever needing that information, but it sounds like Obscura is just using regular WireGuard tunnels.
Edit: I assume this is not how it works but I’d like a written explanation. There are plenty of ways to only exchange public keys though.