It not so a clear cut thing at all, in a cap league. Yes, it would make sense that parties to a contract may choose to mutually terminate said contract, BUT the SPC has pretty clear terms on which the SPC can be terminated (SPC 12, 13 and 14), and there is not one beep of mutual termination. Meanwhile, elsewhere in the CBA it is made pretty clear that you don't alter the deal after it's done, and there also cannot be understanding of any sort that (for example) a front-loaded SPC would be mutually terminated after the bogus years at the end side of the contract has been abused to drive down the AAV. It's anyone's guess whether this completely prohibits the re-negotiations to terminate a SPC:
Do note: even the "no renegotiation" paragraph only has the exception for specific trade clause situations, but does not explicitly allow for mutual termination.
There are ways to get out of a SPC, but per the public rules one party have to first breach the SPC (for example by not reporting to AHL) so the other party can then trigger the one-sided termination process. The parties can obviously explicitly or silently agree to do this choreography.
You're right that it would be a rare occasion because of the obvious reasons re: the salary. There may be something in the non-public NHL By-Laws or some case law if an arbitrator has ever had to look into such a matter. The very least they're very singular cases and the Commissioner may want to be very hands-on if such a thing is ever tried.
