I decided to take a few hours to digest the info that got added after I went to bed last night. After doing so, here are my thoughts:
1st, a lot of the focus has been on what you want and not on what your daughter wants. I am not talking about her impulses and/or fantasies but her actual long-term goals. Does she have any, and if so what are they? You say she is smart so surely she must have some idea what she wants to do with her life and how to pursue it, even if she doesn't have the executive function to pursue it on her own.
2nd, why is she so reluctant around berth control? That isn't something you can make her do if she doesn't want to but why doesn't she want to? Does she want to get pregnant? I can understand someone with impulse issues not caring in the moment, but it sounds more like she is actively avoiding it.
3rd, it doesn't sound like you have a principled objection to her having sex or masturbating, just doing them in a risky manner and/or to the extent where it is getting in the way of her life. That being the case, you might have better luck perswading her if you make it clear that you aren't going to try and impose some Victorian standard of purity on her and work out an agreement that still allows her a reasonable level of freedom in that area. IE unlocked in the evening/overnight when at home and free to go out on weekends with her boyfriend, possibly contingent on getting her grades up/keeping up with coursework.
4th, It seems entirely reasonable for you to insist that if you are going to continue to pay for college she needs to have a system in place to make sure she puts in the effort necessary to get benefit out of it. Insisting that a chastity belt be part of that system is not. You can offer it as an option but it shouldn't be the only one.
5th, as you will see from the discussions on this forum, a chastity regime is a pretty involved thing. Between hygiene, ensuring physical and emotional health, and the various frustrations and distractions caused by the belt itself, it might very well be something that someone with her issues can't handle.
Finally, if your daughter's issues are as severe as you make it sound, I have to wonder if college is really something she will benefit from. If she is unable or unwilling to put in the work needed to do well in school she will probably have just as much trouble holding down a job or doing anything else with her education after she gets it. Without knowing the specifics of her diagnosis, I don't know how realistic it is to expect her to eventually get control of herself to the point where she can manage to live in a neurotypical world but that certainly won't happen until she wants it. Also, it doesn't have to be a permanent decision. She could leave college now and come back to it in a few years when she is better equipped to handle it.
Edit: I thought I might add a bit of relevant humor that might inform others about my perspective. https://preview.redd.it/the-duality-of-my-mind-v0-47jdeyc2ogaa1.png?auto=webp&s=40eb7742a9c00d757d801d8a7db8ae8d7f8e2c63
