Try that Disability office. There are rooms available that you can't find online. Didn't used to be but now that they have these weird algorithms in place you never know. GS CM (not the first level CM) can find things sometimes. I'm thinking that Disability office can too
Tell them the issue from step 1, how you asked to book an HA room and the first CM didn't do it
Have them look all over for options in the other resorts that will fit all of you.
Having the monorail isn't the end all be all either. It breaks down a lot. Then you are left with buses even to MK
You may have to decide which is worth more to you, a monorail to MK or a room your
ECV family members can use and rooms for you all together.