[sara masetti]"So, I compressed all the raw footage outside fcp into prores proxy files (via Mpeg Streamclip)"
[sara masetti]"I was doing File>Media Manager>Create offline AND set sequence: Apple prores proxy"
That's not what you do. The CREATE OFFLINE is for when you use LOG AND TRANSFER (or Log and Capture if you used tape) to bring in the footage say as ProRes Proxy (low res), and now want to bring it back in as ProRes 422, or HQ. Create Offline sets the quality you now want, and allows you to bring in the footage at that resolution. But since you didn't bring in the footage via L&T originally, it won't work.
Just try to reconnect the footage to the new ProRes Proxy files...via RECONNECT MEDIA. Take the H.264 files offline...that might help. But unless the timecodes and durations match exactly, it might not work. But, worth a shot...as the original H.264 each start at 00:00:00:00, and if you used Streamclip, should stay 00:00:00:00. See if that will reconnect.
If it does, GREAT! Then you need to make sequence settings that matches that footage. The current sequence probably is set to H.264...so you need to make a new sequence that is ProRes Proxy (make a new sequence, cut a clip into it and let FCP change the settings), then copy and paste the old sequence of footage into the new sequence.
But this is a very backwards way of doing things, so there might be some hiccups along the way.