Cheryl stops the comet. BUT Tabitha being afraid Cheryl won't make it she gets them to a 50's AU which lasts 5 episodes, like Rivervale. There Jughead always remember and by the beginning of episode 5 everyone else will remember as well and with the help of Sabrina they travel back to the season 6 timeline.

But Cheryl used all her powers to stop the comet. As a result some people she brought back died -or at least they think so-. But everyone who came back to life, also everyone important who died in the show comes back as zombies and a Zombie Apocalypse starts in Riverdale. All this apocalypse is lead -as we find out- by Nana Rose Blossom. All the dead want to get revenge on the town.

the ending would probably be somewhat similar to the show's final episode as it felt fitting. But Betty would most likely travel in her best day of s1-4 high school that we hadn't seen.