Since your daughters are away at college (you call yourselves empty nesters), so I assume they are not living at home and going to classes, and they both have had experience in living with roommates in a community-type situation. That's designed to teach them to get along with others, share small quarters and a community bathroom, etc. So there's no excuse for the sophomore to say she won't share a room with her sister on breaks. She sounds a bit immature but maybe she is retreating into her childhood when her father was healthy and she didn't have to face the adult reality of illness and amputation and wheelchairs. She needs help with the reality of adult life, so counseling is fine, but her demands are not.
If the girls are in fields that will not support them, they need to reevaluate their majors. If grad school is essential, they they do what everyone else does: they get jobs, they get loans, they go to school part-time, they work as residence hall advisors in schools with both graduate and undergrad students, they become teaching assistants, or they live in 4th floor walk-ups with bad paint jobs and tiny kitchens with some other kids. Or they work for 2 years, then go back to school. But they don't demand that you stay in a house you cannot manage.
I vote no on the town house with stairs. The green building sounds wonderful, but you need a ranch-style design with wide doors for a wheelchair and things like a low-mounted microwave (below counter) and an area to pull up to the sink in a wheelchair. You need a place with groundskeepers and no homeowner maintenance. There are a lot of over-55 communities but some may have exceptions for the disabled. Some of the homes might have a lower level that you could make into a family room/studio apartment for your daughter(s) when they visit, but which your husband wouldn't need to go into on a daily basis.
You might do well in a community with activities too - a friend of mine lives in a "co-housing" arrangement where they have small apartments, large shared space for big parties, shared maintenance equipment & tools, and a monthly community dinner. It's a way to meet people but have your own space.
There are also a lot of services (particularly run by women entrepreneurs) that help couples like you organize for downsizing, and also stage your house for optimal sale price. They are usually worth the money. I've met several in my women's networking groups and they are usually caring people who understand your situation. They expect a fair price, of course, but they can give a lot of helpful advice.
I think you make this move for yourselves. If your daughters hate it, maybe they won't come and live off you. If they can't live on their own, they can pay a small rent to you and/or live communally with friends. All my son's friends live in apartments with others their age (early-mid 20s) or in 2-3 family homes that are rented out to young professionals and grad students. Some of them have spent time living with parents (ours has done so, when he first got out of school, and when an apartment fell through and he needed some "emergency" housing). But he pays rent or some other subsidy toward food - we work it out depending on how long he's going to be here. But he very much lives in OUR home and deals with our schedules. He doesn't loaf around monopolizing the TV or expecting dinner on the table every night. He helps out a lot around the house too - he happens to be good with repairs, and he's always willing to help with moving furniture around. We play to his strengths, but he works.
You have added pressures with your husband's health. I really hope he is stable for along time, but you need to look at the stress and expense of moving twice in the next 5-10 years. That's a lot to take on, especially when most of it falls to you.
Time for you to expect your kids to step up. I think they expect to be adults when it comes to their own freedom, going out to bars or with friends, or having a big screen TV. That means they also have to be adults when it comes to Dad's situation, medical bills, financial realities, etc. They don't get to pitch a fit at age 20 because they aren't getting their way.
If they are, in fact, living with you, then you are not empty nesters. So you find a very small 3 BR ranch with an open plan for your husband, with no steps anywhere, including the entry way and the entrance from the garage. You hire a teen to mow your lawn and find a retired guy who works as a handyman as needed. And your kids kick in rent - they don't keep living with you for free on the 4-years-of-college, 2-years-of-grad-school, and the I-can't-find-a-job plan.