[Michael Colin]"1) I'm posting from P2 cards on a MacBook Pro, using the DVCPRO HD 720pn24codec. I can go to a DVCPRO HD deck with FireWire, correct?"
Correct. But you won't be able to output your timeline time code. You can only do that with an HD capture card.
[Michael Colin]"2) What are my choices with the MBPro to get to HDCAM or D5? Just the IO HD?"
That and the Matrox MXO. Only options for MacBook Pro.
[Michael Colin]"And can I output to those decks from my DVCPRO HD 720pn24 timeline, or do I need to up/cross convert?"
Depends...what are your delivery specs? You can output 720p 59.94 just fine, and 720p2 23.98. But if you need to output 1080psf 23.98, you'll need the cross converting capabilities of the I/O HD, or you will need to render it out on a ProRes timeline, then output.
[Michael Colin]"3) Would the results from a DVCPRO HD 720pn24 to HDCAM/D5 conversion via Terranex give me results comparable to going from my timelime to deck via hardware (like IO HD)?"
Yes. I output my DVCPRO HD 720p 23.98 timelines to 1080psf 23.98 HDCAM via the Kona 3. But please bear in mind that an HDCAM and the D5 deck are pretty complex. Unless you know how to use them, I'd recommend not doing yourself. Take your project to an FCP post house with access to HSCAM or D5 and the engineers that know them, and have them output. That is what I do.