If I delete a video from my computer after I've imported it, will it delete from the video I'm editing?
According to the on-line manual page 57 of this commercial software this is only true if you:

have selected the Copy media with project checkbox and
Clicked the Save button

for all your projects!
If you forgot to do that with just one, the answer is:

No, deleting the original file will delete it from all of the projects where you did not click that option.