Am I reading correctly that Rider Switch at
Disneyland lets the too-short kid go through the queue with the parents and big kids, and then the too-short kid waits by the exit with one parent?
No. It's the same as how WDW does it. Under-height kids do not enter the line.
It's the entrance that's confusing when using single rider.
Agreed. And, of course, it's the exit. But exiting is easy.
RSR *can* be tricky IF they send him to fill in space at the HA area. But with RSR we just have everyone exit completely and "wait at or near Stanley but don't photobomb anyone accidentally or on purpose". And of course find Stanley before you enter the line.

That CAN be a long SR line (though it'll be shorter than standby). I wanted my son to be able to wait, be patient, smile at people and perhaps be sociable, before going on RSR.
We actually practiced ahead of time. Well our first practice was at Universal Orlando, where it's not age but being at least 48" (and the height for the ride). We had express pass and we let him go a little crazy with rides we didn't want to ride repeatedly (Woody Woodpecker's coaster 15 times in a row, for instance). Then we went to DL and sent him on Gadget's go-Coaster alone. We sat away from it so we could see basically the whole line. Watched how he behaved, if he cut (accidentally or on purpose), if he was sociable and kind, if he looked scared vs brave, etc. He passed the test, so we moved on to RSR from there.
For you I would suggest letting him take the lead in line and getting everyone seated on rides you all can do before hitting SR lines. Let him show you where the entrance and exit is, how to get into line, tell them how many are riding, etc. That gives him practice in being in charge and lets you know he can pay attention to the line, the crowd, AND the CMs, and he might be more confident when going alone.