[George Snow]"He transcoded the DSLR footage to ProRes 422 (HQ). Which again does not play. "
[George Snow]"Since I've transcoded files that were already transcoded, have I lost any quality? Is this something that shouldn't be done? "
Whenever you transcode to a compressed format there is some quality lost, although in this case I wouldn't be concerned.
What you have to fix is the issue of the Prores HQ not working on your system. Should do it.
If you don't have the original cards or a copy of them your only method is to transcode from the ProRes HQ to ProRes which is suitable for FCP 6. Any very minor quality loss will be negligible.
Transcoding HDSLR H264 footage to ProRes HQ will not improve the quality, it will just give you larger files, ProRes SQ is as good as it gets. Even ProRes LT is suitable a lot of projects (if you are using FCP 7).
All you get with ProRes HQ is larger file versions of an already quality compromised H264 codec. Transcoding to ProRes HQ is a bit like painting a Ford Fiesta a Ferrari Red colour and expecting it to go faster.