Most attractions do have an option for this situation, it is just generally discouraged and not advertised, so you may have to insist on the alternative.I often travel solo, and wondered this also. But I've had CMs offer to push me 3 times-twice at Pirates and once at Pooh. One time at Pirates, it was a "suit" who offered to push me, then he gave me 3 anytime FP+s (yes, a few years ago) . So I don't understand the "Cast Members cannot push guests"
Rides I know that ECVs are not allowed through the queue (not even the FP queue), but have alternative options where you wont have to use a wheelchair:
Buzz
Tower of Terror
Rock n Roller Coaster
Test Track
Haunted Mansion
Not sure about other attractions.