As you have already found out, video is the biggest consumer of disc space.
I would buy a large external HD, at least 1TB but 2TB or more would be better.
This new drive needs to be formatted as Mac OS Extended (Journaled) - (via Disk Utility) and HAS to connect to your computer via firewire - not USB.
You can then keep all your video files on this external drive, they never have to be on your internal HD.
There are instructions in iMovie Help on how to move video to an external disc.
You could partition this drive and use a portion of it as your time machine backup.
If you have totally finished with your video files and never want to do any more with them, then certainly you can trash them and then Empty your trash.
If you imported all of them into iMovie then you now have 2 copies although iMovie may have altered them depending upon your preferences.
I always keep my raw unedited files on an external drive in case I ever want to re-use them but that is up to you.
Events can be deleted from iMovie ONLY after you have exported your project otherwise your project will no longer function.
I never compress my movies, I always leave them at the same size at which they were shot.
Why would you want to do that? If it is to save disc space then I consider it to be false economy.
Yes time machine makes a back up of your files but that it not what I was talking about.
Either get a new drive or partition your existing drive (which would have to be wiped first).
Have one partition only for time machine and the other for general purpose specifically for working with video files.
Import your video either directly to this external drive or through iMovie and do all your editing to and from this new drive.
That result is that video never is stored on your internal HD, so you should never run out of space on your HD.