Regarding Andy and Fred -- I don't know what Michael O'Keefe's willingness/unwillingness is to return to this franchise -- but they could easily bring back Andy without necessarily showing us Fred. If O'Keefe was unwilling to return, Fred could have just died offscreen. Although if O'Keefe WAS willing to come back (assuming that he and Laurie Metcalf enjoyed working together), that could be intriguing too.

In the episode where Darlene learns of Molly's death, Molly's funeral is never shown to us. We only see Darlene returning from having attended the funeral. So, in theory, Charlotte could have been there, just offscreen -- however, I prefer the concept that Charlotte skipped the funeral due to her offscreen estrangement from Molly...and Charlotte returning onscreen could be prompted by her wanting to check in with Darlene, since Darlene was one of the last people to see Molly alive (unlike in the original series, where Darlene and Molly had a consistently adversarial relationship -- in "The Conners," Darlene and Molly seemed on their way to becoming tentative friends after almost 30 years of noncontact).

(Also, Ty Tilden doesn't appear in the episode; Molly's "mother" does -- although it's unclear if she's Molly and Charlotte's stepmother due to Ty having gotten remarried offscreen, or if she's their birth mother with whom Ty got back together, once the Tildens had disappeared from "Roseanne" -- I don't recall whether or not Ty was explicitly said to be divorced/separated or widowed, in the original "Roseanne")