Thank you for your questions.
Sorry to hear about the health challenges that prompt them.
The text of the ADA and court decisions are very plain that the ADA does not apply until and unless the employee
or customer has a condition which "materially" or some similar term interferes with the activities of daily living. In the employment context, it expands a bit to include the bona fide physical performance requirements of the assigned job position (my words, trying to make it understandable).
Once that threshold it met, an employer must provide "reasonable accommodations" for the disability if at all possible. A company servicing cell phone towers, for example, most likely would not be required to accommodate a person whose knees now prevent climbing ladders or the tower dog position, but might be required to find a desk job *if* one is or becomes available.
Under the employer-sponsored health insurance
plan rules, it is illegal to fire someone because they cost the insurance
plan too much. The separate subject of wrongful termination is pretty broad, but know that once an "at will" employer decides to dump someone, they usually can use either no reason or any reason if they need to cut WAGE payroll costs (layoffs, for example), if they are clever enough to not let any evidence of their true discriminatory intentions get generated (saying things out loud, sending memos or e-mails with "smoking gun" comments, etc.). "Pretext firing" is very hard to prove, but enough employers are arrogant and careless enough that such cases do get won by employees once in a while.
Time off for medical visits is covered under federal law, Family Medical Leave Act, if the employer is big enough (50 employees IIRC, can be aggregated among different locations if they are close enough together geographically).
The ADA generally does not apply to illness time off--that's FMLA if applicable. However, regular visits like dialysis trips could force some adjustments to the work schedule because kidney failure requiring that does interfere with activities of daily living.