Apologies for taking a while to reply but I've been in China for the last 3 weeks without access to any Google-related services thanks to "The Great Firewall of China".
The issues you are describing could be due to the presence of the "Cycle Realistic" (or, "Cycle Accurate") functionality in the latest release, which makes both Sing Step and Sing Inst switches on the front panel behave as on a real PDP-8. The behavior for booting different OSs and mounting USB still exists, it just takes a different action to make it happen. Here's how:
*) Mounting USB media and booting SIMH to other OS's does work, it's just a different switch sequence than before. Put the desired DF and IF same as before, only now (in this order):
Sing Inst on
While holding down Cont
Toggle Sing Step
If you are mounting USB media, you will need to set both Sing Step and Sing Inst back off and then hit Cont to get running again. If you are booting SIMH to a different OS, you might not need to hit Cont. If you're too slow setting Sing Step and Sing Inst back off then you would need to hit Cont.
*) Rebooting the Pi is:
Sing Inst on
While holding down Start
Toggle Sing Step
*) Shutting down the Pi is:
Sing Inst on
While holding down Stop
Toggle Sing Step
Hope this helps,
-- steve