These are really good questions, but/and I think there must be some guidance for them already out there. I did a little Googling and found this:

https://www.eeoc.gov/laws/guidance/enforcement-guidance-reasonable-accommodation-and-undue-hardship-under-ada

https://askjan.org/a-to-z.cfm

https://odr.dc.gov/book/manual-accommodating-employees-disabilities/types-reasonable-accommodation

https://askearn.org/page/reasonable-accommodations

One of the reasons I was doubtful about this question is that it started out at the position that accomodations are/can be impossible. I think starting out at "this will be impossible unless proven otherwise" will end up being a self-fulfilling prophecy, you know?