Let me break down this workflow for you.
There’s a few things you can try. For the neck issue with the mask, you might try isolating and patching from a section of background behind the talent and then rotoscoping back over the top to get rid of that issue. You might be able to take a large section of the BG and feather it out behind her. And correctly roto everything in back on top.
Barring that, you will have to do a stabilized paint series of precomps.
Degrain your footage.
So first thing you would to is track the problem area with everything including perspective and use adjust track if you have any slipping. Then use the stabilization module with “all motion” and “maximum smoothing” to lock it down completely. Use a CC Power Pin corner pin exported from Mocha to lock it down and make sure it is inverted to stabilize the frame completely. Copy the corner pin effect or the Mocha Pro effect so you can rewarp the patch later on.
Precomp this stabilized clip and move all attributes into the new composition. Paint on top of this with your AE Paint tools, you may need to use some roto to make it work perfectly in this comp. When you are satisfied, precomp THIS painted precomp and move all attributes into the new composition.
Now apply your corner pin back on top or use the Mocha Pro effect to apply the corner pin back on top and rewarp the footage. You may need to use a track matte with feathering to make it blend nicely.
Regrain your footage to hide any minor problems.
Hopefully this works!
Have a great day,