Hi
"Separate question: What is permitted while the progress indicator
in the lower right is buzzing? Must one simply wait?"
No, not necessarily. You can do most things whilst the progress bar is playing because VP is assembling your project or loading effects between the clips at this point (as shown by the completion of the blue bar over the sequence images or the yellow bars over the effects blocks,) You can play the sequence up the place where the red cursor bar reaches the uncompleted yellow or blue progress bar at which point the preview pane shows the message "Building Preview Please Wait"
If you need to split the clip before the effect has been completed, you can do as the effect simply carries on (or restarts) from the start of the right hand section or continues to finish at the split point.
However, it's probably best to wait until all the actions are complete before doing another edit.
Nat