Sharing a locked room is a showstopper
This is a person: whose mental stability and reliability is unestablished; who has difficulty resolving personal differences in drama-free ways (and perhaps even a penchant for drama); and who also seems manipulative, and who seems unconcerned for your situation, i.e. Sociopathy. I'll grant you this is a "glass half empty" view, but we're doing risk analysis here so that's necessary.
You mention this, but I don't think you're treating it seriously enough. Risk is chance x severity. Low chance, but severity is the end of your life as you know it: scandal, unemployability, poverty, and even prison and sex offender registry. An easily made accusation by her could result in a big payoff..
We have a rule at my employer that an employee can't be alone with a customer (i.e. 1 of each). Guest because they are not known quantities, as is not a new hire. So our office would never allow someone to be put in that position.
This isn't really believable as an ADA accommodation
If one person really needed dark, locked door, they'd handle this by tasking a "mother's room", maintenance room, utility space, or small meeting room for that person alone (or birds of that feather).
The company might even poll around for employees who want to work in a dark, closed-door, silent, NOC type environment.
Or they could simply let the person work from home, citing ADA as the reason and since they won't be talking to anyone anyway.
Regardless, ADA compliance is not your job. A rank and file employee can't use ADA as a club on other coworkers; that's always bull, and is not a bona-fide ADA claim. The workflow is: the employee takes the compliance request to HR... HR and legal confer over whether this is bona fide.. HR and management confer with employees on whether and how to accommodate... And at that point you raise any objections to where hte plan might impact you. You can also raise those objections later.
Besides, nevermind ADA. The Building Codes require a minimum level of light in employee office spaces. An individual employee can opt out if they like (NOC etc.) but they can't impose it on other employees. So this is kind of an ADA matter after all, if you can't see to work. Vision is definitely an ADA protected class.