How to regenerate an overstretch fin and skin damage by shrimp of a stuck molly?

There is not a lot one can do when the fin is torn, other than keep an eye on it for any infection and start treatment if it gets infected.

I have a pleco that got a torn fin; it happened when I moved the fish to another tank about a half year ago. Within 2-3 weeks, the fin was completely healed (the fin was completely split in two, so I did not have any hope for it to end well).

I know this is a bad answer, but there are really no good options on how to treat this problem (at least that I know of).

My opinion would be similar to @trondhansen in the way that- there's not anything you can do to fix, BUT I'd say there are things you can do to help make your Molly either more comfortable or to prevent infection or to help with any scales that may have fallen off in process to regenerate. Adding a dose of fresh water aquarium salt helps with scale reproduction, and would be soothing to any tears. There are ways to clean it BUT your fish has gone thru something VERY stressful, so for that purpose I would stick with a antiseptic tank additive. One that I can think of off the top of my head is made by Seachem called "Stress Guard"... Added a pic of it. But yes, as previously stated, your fish may "heal" but not go back to normal.

Also, aquarium salt is my "go to" for a lot of freshwater fish issues, but it is an especially safe additive to use in Molly due to how salt tolerant they are and that they can be acclimated to brackish and saltwater (although I don't recommend doing this if you don't know what you're doing) I felt it was a good point to add. Good luck, she's pretty.