I'll try and shed some light on the single rider debate.
Disney prefers using single-rider when a ride meets all/most of the following criteria:
1.
When the ride has heavy demand for it. Single rider takes some resources to run, including extra staffing, and space in the queue that could be used for normal guests otherwise. Disney likes using single rider when an an increase in the ride's throughput is desperately needed (aka really popular rides). If the ride has a 10-20 minute wait most days... they probably won't bother.
2.
When vehicles have 2-3 seats per row. With wider vehicles (think Safaris or Soarin), cast can more easily arrange parties in a way where there's no empty seats. With narrow vehicles, there's a higher likelihood of having single seats going unfilled. 3 seats in a row are especially awkward (think Test Track), since most parties come in multiples of 2, believe it or not.
3.
When vehicles are dispatched quickly. This is a big one. Cast don't always have time for fancy mental math or shifting parties around, they need those seats filled ASAP so they can send the vehicle through. Single-rider is a great way to do that (again, think Test Track or RSR. Very fast dispatches). Compare that to Soarin' or Safaris again, which may have several minutes between vehicles; in this case cast have much more time to fish through the queue to find that party of 3 or whatever they need.
4.
When there's space in the queue. Another big one. Many rides don't have space; queues can fill up quick, and carving out a 3-foot walkway for single riders to walk straight to the front of the line can eat up precious queue space. You'll notice most rides with single rider have huge dedicated lots of land (Everest, Test Track, Indy). You won't be seeing single rider in Fantasyland rides anytime soon.
5.
Doesn't make it completely and totally awkward. This one has been touched upon already with the Haunted Mansion example. DCA had single-rider for TSMM for awhile, since it matched all the other criteria. But the fact of the matter is that some rides are a little more "personal" than others. No one's making conversation while screaming on Everest or RnRC, but on rides like Mansion or TSMM you usually prefer to know the person next to you. So DCA removed single rider from TSMM.
Hope that helps
As for the "seasonal" QSR thing, I only with I had an answer.