How to follow up with a master's advisor who abandoned me?
This is a peculiar situation. Your supervisor did not honor your agreement, but at this point she has nothing to gain from continuing to be involved with your thesis, and being angry about that isn't going to help you in any way. Realistically, do not expect any more input from her, and move on.
Getting someone else to look at your thesis could be useful, but keep in mind that you are asking someone to put in a couple days of work for nothing in return. Try to find someone who could be at least slightly interested in this, and see what you can do to minimize the effort for them. Maybe approach a lecturer whose class you took during your degree, or enroll in a new course if you think that you have burned bridges in that university? Search around if there is some other researcher, even elsewhere in the world, who has published on a similar topic -- in particular, someone early in their career might be more excited to help.
Even then, clean up the writing as much as you can by yourself, maybe send them just the key proofs, and ask targeted questions -- in other words, approach this always keeping in mind that you are asking for a favor -- and you are more likely to get a reply.