File/Options/One File → Sequence is a result of an optional package that is loaded, and
Options is only filled out when a package places a menu there. I’m pretty sure newer RV versions remove that menu by default until a package places something there to avoid the confusion.
This is indeed true. If you set that environment variable to a very high number, you’d get the behavior you are looking for, but it would affect everything (so you’d effectively never be able to load in a sequence).
Your best bet would be an alternate scripted
File>Open Images Without Sequence.
You could essentially use
commands.existingFilesInSequence to determine what frames exist, then run a
commands.addSources with each frame specified as a sequence individually and add the
+noSequence modifier (which is the -noSequence`)
This is the analog to doing
rv <file1> <file2> <file> ... -noSequence