An update: I fixed problem 2 by changing the last line of /etc/sddm.conf.d to:
RememberLastSession=true
Problem 1 persists, and sounds related to https://bugs.kde.org/show_bug.cgi?id=445449 . If from the terminal I issue:
/lib64/qt5/bin/qdbus org.kde.ksmserver /KSMServer logout 0 0 2
the system goes promptly to the logout screen, and allows a normal, speedy reboot. If on, the other hand, I change the command to:
/lib64/qt5/bin/qdbus org.kde.ksmserver /KSMServer logout 0 1 2
to instead invoke reboot, it gives me the ‘System will reboot now’ message, and delays ninety seconds before reboot. (Not two minutes. This time I clocked it.)
I’m now on sddm 0.19.0-11.1, brought in with the 20230425 snapshot.