I can't find the thread eight now to point you to the documentation, but there is no standard in the current ADA Guidelines for bed height in a wheelchair accessible room. The current standards only give guidelines that must be followed for items that are permanently attached to the room, such as sink and toilets. Those Guidelines do not address any removable things like beds, chairs or other furniture.
So, unfortunately, the resort is correct in telling you their bed meets the standard because there is no standard to meet.
(Bed height IS part of the new draft standards because of just the problem you are having. Once those standards take effect, the bed height will be set the same as toilet height.)
I can't see whether there is a platform under the bed or if it is open. My advice is to call Housekeeping and tell them that your bed is too high and you need the mattress placed on the floor (check first that would not be too low for you). You can also request a bath chair if thst would help you with the shower situation. The bath chair can be placed into the tub, you sit on it from outside the tub and then swing your legs into the tub. Those can be requested from Housekeeping. My one warning is to look at it carefully before using it. We got a wobbly one from Housekeeping in October at OKW.
About the other things -
room with roll in shower - that depends on how the reservation and the request was made. If it was truely a request ("I would like a room with a roll in shower",) then it is just a request that may or may not have been met. If you said you needed one, it should have been guaranteed. When you made the reservation thru Disney reservation for a need, they should have checked with the department called Special Reservations to make sure there actually was a room with a roll in shower available for your dates.
Then it would have been blocked for you and should nit have been 'given away' to another guest. If you went thru a
travel agent, you have no way of knowing whether they listed it as a need or a request.
Room location - the current ADA Guidelines ( and the draft ones that are nit in place yet) say that handicapped accessible rooms should not be segregated in one spot, but need to be distributed around the resort. Because if that, some handicaaped accessible rooms will be farther away from everythng and some closer. The location is a request that may or may not be met.
I would encourage you to keep some notes about your experience and contact WDW after you return home.