Maddy
Where is Maddy since she was revealed to be in the Black Lodge at the end of Season 2? Also wouldn’t Cooper preventing Laura’s death also prevent Maddy’s death too?
shareI think there are a few "where were they's" for season 3 (most notably Annie in my opinion, or even Donna). But given the focus that was season 3 on Cooper's journey I can understand why these other characters weren't explored. You can have only so many cameo's for fan service sake: Mike Nelson for example.
As for your second question: Yes. Absolutely.
Unless one imagines Laura having vanished and so Leland turns his obsession toward Maddy and the same result occurs.