Here are some answers for you:
1. H.264 files off of a Canon 5D need to be converted to ProPes 422 in Compressor of MPEG Streamclip before you can edit with them..
2. Panasonic and FCP work together beautifully when going off/online..
3. Yes you’re right XDCAM does not support the Offline/Online workflow in FCP. What we have done is to use SCRATCHLAB to generate proxy files for offline off the cards, then export an EDL, and recapture in Online Res.. complicated, but it saved the production…