Or vice versa? If so, how can this be achieved?
Is there a way to route a mullvad connection through another vpn provider?
That is very easy to do, I have done it for a few years and it works like a charm.
I have Proton VPN installed on my router and Mullvad VPN on my devices. It’s like Multi-hop, in this case Mullvad VPN uses the Proton VPN connection. Sites will always see Mullvad VPN, not Proton VPN.
Yes, establish a vpn tunnel with a Mullvad peer on a VPS, and create an VPN service on the VPS, then connect to that VPS, you can connect the VPS’s VPN on your phone, PC whatever. The nested VPN was running on the VPS not your client side.
Definitely Tor browser
maybe directly Wireguard or OpenVPN supports it
not via your phone, you can set it up on a router 
use a vpn router from gl-inet then use software on what ever device
Fragmentation will kill your internet performance due to encryption headers.
Use the VPN along with TOR.
cant imagine why a “normal” user would do something like this
Just run tor ontop of a vpn at that point if you are that worried. Probably won’t be the fastest thing ever.
Is it possible to do on a mobile phone that’s connected via 5G?
Why? Because the likelihood of one supplier getting compromised is a lot higher than two.
Ah, I see, you are talking about one connection being compromised. Then Obscura is the way.