I have no insights into the location of the temp files, but here’s a workaround: Create a new user account. Log into the new account (quit iTunes first if you use fast user switching) and launch iTunes. You may need to authorize your account in iTunes.
Try the download, and if it is successful, right-click in iTunes and select “Show in Finder.” Drag the file to /Users/Shared. Log out of the new account and back into your main account, and drag the movie from Shared onto the iTunes account to import.
Creating a new user account is a handy troubleshooting technique. The new account will have pristine preferences, caches, etc., which can help narrow down all kinds of issues.
I didn’t notice before that your iTunes library is in the Shared folder. If you use a new user account, do NOT link it to the iTunes library in Shared — create a new library (which should be the default option on launch), to make sure you’re not inheriting any cruft in the old one.