Can't the contract be voided if both parties agree even if it's determined by NHL it was fully executed?
In normal circumstances, I would guess yes.
The hiccup scenario in my eyes could be if the parties agree (maybe even before the signing) that they will not respect the 2-way element of an ELC, but that they will void the contract instead in case the player doesn't make it to the NHL roster.
That ELCs are 2-way, is set in the CBA. NHL wants their cheap reserve players, and won't look kindly if the teams actively try to circumvent the ELC=2-way rule.
By the CBA, if a player don't want to go to AHL on AHL salary, he can require an European Assignment clause that allows him to find an overseas team instead.
The player also has the "option" to not report to the AHL team, though that's a contractual breach on player's part.
Obviously the player can be traded to another team.
But delibatery agreeing to void a SPC so a player can go and sign with another NHL team where he actually makes the NHL team would be a questionable move and bad precedent if allowed.
Disclaimer: all this is speculation and spitballing.