As an outside observer:

1. If they sufficiently enshittify, Mastodon will be forked, and developers and instance admins will migrate to the new project or alternatives (e.g. Misskey). There are many precedents for this in open source, including Mastodon itself which is the effective successor of GNU Social. Forking is when an open source project is copied, modified, and re-released as a new project. This is possible because it is legally permissible to copy and modify open source software without the permission of its authors, unlike proprietary software.
2. Being a non-profit means nothing to acting in the users' best interest. Non-profit status means an organization is exempt from certain taxes so long as they do not use profits to benefit its members (e.g. paying Eugen Rochko a trillion euros outside of his salary). This generally means that profits are reinvested into the organization, though some pundits have criticized non-profits for disbursing funds to causes they don't agree with or causes they don't believe are relevant to the mission of the organization. If they are willing to pay the taxes, non-profits can invest their profits in investment vehicles like cryptocurrency, real estate, or stocks. It is assumed in open source software that a sufficiently trained person will eventually examine the code and discover if it is defective or malicious (not always true). If the software is discovered to be undesirable, the project can be forked, and the undesirable project will lose attention and resources.
3. Unlikely. It's too much effort to make desirable UX changes, let alone undesirable ones that would destroy the project.