If the JW religion is the only one that saves, then mentally and physically handicapped won't be saved.
I'm not sure what gives you that idea. Severely disabled people are always in the minority in any group. I've known people in the organization with a variety of disabilities who were respected just as much as anyone else, even a MS who was one of the most active members of the congregation despite being in a wheelchair.
Besides, look at any WT illustration that depicts the aftermath of Armageddon, and you'll see people throwing away crutches and white canes, leaping out of wheelchairs...
(I have to admit though, I was never quite sure about those with very severe mental disabilities. I just figured God didn't have to follow the WT's salvation criteria to the letter, and would do the right thing when the time came.)