Hey Jonathan,
Whew that's a load of great questions for sure. OK lets get started.
1) What you say is correct in that it records HDV (4:2:0 MPEG-2) to tape and through firewire.
2) The image is VERY good with the JVC codec (19Mb/s)
3) Keying is similar to any of the cams in this price range (sub $10k)
4) You will overall yield similar results with all the cams you listed (EX, HVX).
Since you have the kona you can capture from the uncompressed HD-SDI out of the JVC HD250 to what ever codec you like in the kona (via live feed).
In my experience the JVC codec is an adequate acquisition format, not a great post format. I usually convert to ProRes422 and go from there. If it's just a simple color correction and down-convert then it depends. The paper says one thing the image says everything.
The file size is smaller than a DVCPRO file of the same length. I don't how they did it but it rocks the house. My advice would be go into your 'try-out' with an open mind and run the hd250 through it's paces and see if it's a fit for you and the company.
Make sure you load up the DSC CDM28 and SUPERWIDE settings from Tim Dashwood:
http://dvinfo.net/conf/showthread.php?t=93052 (link in blue at the top of the page)
Johnny C
