Habitat?
Habitat Curacao Resort - The dive hotel in the Caribbean They're the only AI left mid-island now afaik.
We went to (but didn't stay at) both in 2007. Habitat has a better house reef (Nos Kas) and their dive facilities are on the water compared to the beach hike at Sunset Divers. There was some mention of new owners here recently so I'm not sure of the status of the resort now - when we were there it looked a little tired.
Nice pool area, decent restaurant, it's another vertical property Like SW - 3 flights of stairs from the resort/pool area to the water. Good dive facilities right at the waters edge, lockers, tanks, a good setup area with stand-up benches (you mentioned back issues) with a dive dock with stairs 20' away. I did read recently that the last hurricane took out their longer dive dock which put you just about on the reef - but that may've been re-built by now.
No beach if that's of interest - just a sundeck on the ironshore at water level. It's pretty much the only resort in that area, there's a small housing development on the hill above/around it. Gated security for the housing community extends to the resort.
Another option might be Lodge Kura Hulanda in WestPunt. But it's fancier than you sound like you want - one of the Leading Small Hotels of the World. And quite a bit more expensive afaik. Nice resort, plush suites. (I was in one once) The resort is located a 2min walk from Ocean Encounters West - one of the nicer dive setups we saw on Curacao. They have a great house reef in both directions - it's the Alice in Wonderland/Playa Kalki dive site and all gear is stored in a locked area 30' from their dive dock. Really nice people ran it also.
The other AI's that I'm aware of would be in east Willemsted - Lions Dive or Breezes. Both are affiliated with Ocean Encounters also, their main shop is an easy short (board)walk from the Lions Dive property - they do the Seaquarium Dolphin Dives from there also - and have a satellite facility at Breezes. From Breezes (I've never been there) you'd also have to option to do escorted shore dives with the
DiveBus next door - Suzy and Mark are good people - recommended here regularly.