There is probably a preference in iMovie to copy to library on import OR reference original location.
Quicktime issues? Could be anything. Down res them to something smaller perhaps when exporting to QT.
This is why I quit working as an assistant editor. IT SUCKS. Seriously. Editing is the most soul sucking tedious shit you can do, and you're not even getting paid. Quit while you're ahead.
It's not so bad if you have a capable machine and are doing things in your comfort zone. Repetitive and ball-aching sometimes, but it's not the worst.