I also live in a high rise condo so I understand issues of remodeling in a high rise condo which are unique. My condo also has concrete floors and concrete ceilings and is steel framed - all of which impact construction techniques.

I am completely aware of issues regarding what can or can't be done. When I did a gut remodel which included removing the tub/shower in place of a walk in shower, I had to get permission from my downstairs neighbor to access the drain to change it.

As others have pointed out a completely curbless or almost completely curbless shower requires special construction of the shower and even bathroom floor which will probably be impossible to achieve in your unit. I am not even sure that it would pass Code in a high rise. Many construction materials and methods are not permitted in a high rise multifamily unit. For starters, anything that increases the risks of flooding or even leaks is off limits because of the possible damage to units below you. We have had several floods in my building which have damaged units from the 12th floor to the lobby - owners have had to move out while their units were repaired.

I also have some mobility issues and I remodeled with an eye to aging in place. You can check to find out what the minimum height and width for a curbed shower is. What works for me is to have several grab bars - one is at the entrance to the shower and I have another one that can be reached easily as I am moving into the shower. So even when I had surgery which made it difficult to lift my leg, the grab bars enabled me to safety access the shower. They don't look institutional at all as they match the rest of the hardware.

You should design the shower with a built in bench - absolutely critical if you are dealing with physical challenges. While some might recommend a moveable or fold down bench, you need the stability of a built in bench with a back that isn't glass so that you can lean your weight on it safely. I have two hand held shower heads in addition to the fixed one as I have one installed by the bench so that I can access it while sitting at the bench. It also means I don't have the hand held snaking across the shower and it is very easy to place it back on and take off as necessary.

FWIW my father lived in an assisted living facility which did not have curbless showers. The showers had very low curbs and the residents were able to use the showers.

As others have pointed out, all three options for the shower are wrong and even in a high cost of living high rise renovation, you should be able to get a properly built waterproofed shower for $26,000. Obviously you could spend more depending on materials.

Also as others have pointed out, it is unlikely that the floor will be salvaged under the tub - but in the scheme of things redoing the floors is not going to be the major expense.

First I would see what contractors others have used in your building and get recommendations from your manager. I had my remodel interrupted by Covid and I am starting up again to do some of the finishing work. When I called my manager to tell him, I didn't have to go through hoops since he knew my GC - from my remodel and other work and knew that he was a responsible guy who knew the rules and followed them. My building is extremely strict about the administrative stuff - we and the GC sign an "agreement" where all the rules are spelled out. Proof of insurance naming the building as an additional insured is required as well as proof of a GC license. Technically my last agreement had expired but I was permitted to proceed without going through the red tape again.

I would find a good designer - preferably one who is knowledgeable about the needs of people with mobility issues or just designing to age in place. My designer was very helpful in terms of functional issues. But before proceeding in a piecemeal manner with the "fixes" suggested by all of your GC, I would get someone who understands what needs to be done and how it should be done. I don't know your finances obviously but if $26,000 for the shower is doable for you, it is likely that you would also be able to afford the cost of a bathroom remodel that is done correctly and gets you what you need.

ETA - Ignore any advice about costs from people who are not remodeling in a high rise condo in a metropolitan area with a high cost of living - and for which extensive permits are probably also required as well as inspections of the work as it proceeds. There are costs for condo renovations plus EXTRA costs for work in a high rise that raise costs significantly - for a competent GC and the tradespeople he uses. Protecting the common area was a surprisingly high line item cost. Hours for work are limited and a knowledgeable GC will reflect those costs in his estimate/bid. He will also know that it takes longer for materials to go up and down - so demo costs will be higher. There are scheduling issues - workmen have to wait for elevators. You might have to check for asbestos before any demo is done depending on the age of the building. My building is older so it is required before any work is done.