I think the best solution would be to invite The Mamas to MF 2021 with a new (even better) song.
They should also have a direct spot in the final and could perform the song during the first semi just to "show it" for the viewers, the song would then officially be realesed after the fourth semi with rest of the finalists.
Other than that the competion could continue as usual, so in the end the only difference would be we have 13 songs in the final.
Also don't think you could rule out The Mamas win again, they have the support from the viewers and even more because of this situation.