I still want a spirit to get promoted solely so I can see how they figure out a way to make both versions 'canon'. Like a split soul or something.
Honestly, dlc for many games have largely been known to simply throw current canon out the window.
If it really bugs Sakurai or the development team that much, they'll just have the trailer reveal taking place after the true ending with Galeem and Dharkon. Either by returning to their real world first and receiving the invitation, or simply escaping that ascending spirit twister to return to the Smash world.
As for the spirit itself, they don't even need to cut it. They can take a few seconds out of the way during the development/programming process to just simply opening the file for spirit name and changing it to include the source game in parentheses if it bugs them even further. E.g.
Shantae (Half-Genie Hero)
Rayman (Origins)
The fact Rex was pointed out as being too late for the Fighter Pass as if he was interested in including him and handed not only a Mii outfit, but even gave him a base game spirit, is proof enough these disconfirmations are only temporary and quite likely within Ultimate's roster selection cycle itself. Sure we never had things like an Assist Trophy promotion within the same Smash, but remember, we never had DLC lasting over months.