Question: Is there a reason why Roger, Anita, and their child are not back for this sequel? Also, how and why did Dipstick get adopted by Chloe?
Answer: We are never told why they are not there and we are not told about how Dipstick came to live with Chloe.
Question: Why do Jasper and Horace not appear in this film? Are they still in prison or did Cruella just lose faith in them?
Answer: Most likely both.
Question: Why was oddball stuck in the cake batter and why was she struggling to get free?
Answer: It appears that she somehow fell in and she was struggling to get out because Cruella was there and because the cake was about to be sliced to be baked.
Question: When Cruella reverts back to her former personality why is everything spotty?
Chosen answer: This is due to her obsession of wanting a spotted dalmatian puppy coat.
Answer: Because Jeff Daniels and Joely Richardson who portrayed Roger and Anita were busy filming other projects at the time and maybe they chose not to return. Or perhaps the writers couldn't find a way to fit them into the Story. As to how Dipstick got adopted by Chloe, maybe it's just a mystery we'll never know.