How to handle severe illness in promotion and tenure reports?

(I'm very sorry for your troubles...)

To supplement other comments and answers:

In my experience (at R1 places in the U.S.), while mostly undergrads' and grad students' (broadly speaking) "disability issues" have a roughly appropriate procedure in place, just as an illustrative and ominous example, this does not seem to extend to grad students who've (e.g.) had their leg broken in a car accident through no fault of their own. That is, one of my students had this happen, could not do TA (teaching assistant) duties, and had to petition for "leave of absence" and was supposedly not allowed to be on campus, use the library, etc., but was allowed to (!) petition to return. This policy was beyond the power of my department to alter. I was told that the university is "more caring" about faculty in similar situations... presumably because they've invested more in them. But, my point is, I would be very suspicious and untrusting.

E.g., it might happen (given my observations) that dept head and lower-level deans are sympathetic, supportive, and quasi-promise that they'll accommodate... but then eventually discover that "university policy" prevents them from doing what they promised they'd do. E.g., even a signed document from them may easily (!) be countermanded by higher-ups, claiming that they did not have the authority to make the promises they made.

Yes, talk to the disabilities office. The "Americans with Disabilities Act" has not yet been scrapped (fingers crossed), and U.S. (especially public, in the U.S. as opposed to U.K. sense) have considerable power to set accommodations. Yes, they attempt to "negotiate" with relevant work-place authorities, but the rough idea is that you cannot be fired or marginalized for (new or old) disabilities. Having things in place with the Disabilities Office may be your best "insurance" against higher-ups' disavowing your departments' agreements with you, since they cannot so effortlessly over-ride them. I'd absolutely do this.

And, above all, no "accommodation" should include your losing your health insurance just because you have some periods when you can't teach classes!!! Obviously, in the U.S., anything like that is very, very dangerous for people who are not Magically/Luckily Healthy. The reason I mention such a thing is that my student, mentioned above, did lose health insurance exactly due to the broken leg that made him unable to TA... Luckily, I believe his partner's employment covered him. A bit too unfunny, non-recreational Kafka-esque for my taste.

(Again, very sorry about your increasing troubles, and best hopes for no additional troubles due to bad bureaucracy...)

My advice is that you decide exactly what you feel would be an appropriate accommodation and then check with whatever office handles disability issues for faculty about how to document that accommodation. Unfortunately, many US universities are still behind the curve in accommodating faculty with disabilities, so you may need to be persistent.

Do not "negotiate" with the dean or the chair about this because compromising about your accommodations can be used against you later. I'm not saying that you should be adversarial about it, but any power you have here is now, while they are being cooperative. As Buzz's comment above shows, if you don't get this written in stone now, it will be too late after review.