In a sense the basis of your question is in error.
While iMovie is saving videos to your camera roll, this isn't using any extra space on your iPhone. That's why you get the error you're getting after deleting the files - because that's where iMovie is trying to access the video from.
Basically, what happens is the video is saved to your camera roll, but project specific info is saved within iMovie. Once you have totally finished a project, you can export/share/save the final version and after that you can delete the video you took from your camera roll (if you want).
If you like, you can actually prove this to yourself by doing a simple test:
- Go to Settings > General > Storage & iCloud usage
- In the Storage section, tap on Manage Storage
- Wait for the list of apps to appear (this may take a while if you have lots of apps).
- Tap on iMovie (likely to be in your top 10)
- Take a note of how much space is used up by Documents & Data
- Now go to your actual iMovie app and take a 30s video
- Go to your Camera roll and try to email this video to yourself
- You should get a warning saying that this attachment may be too large to email. This message will give you the size of the video. Take a note of this.
- Cancel the attempt to email the video.
- Repeat the first 5 steps.
- You should note that while your iMovie Documents & Data have grown a little, it will not have grown by anywhere near the size of the actual video itself. The reason this has grown is because it's storing project info, not because it's storing the video again.
Hopefully this little test will put your mind at ease.
Don't forget to delete your test video.