HDCD Transport to non HDCD dac

No. If you are using the digital out the quality will be the same as redbook.

No, but I wouldn't base a purchase decision on it. Its nice to have, but its not that popular and doesn't make that big of a difference. (At least for me.)

It does make quite a bit of difference, I have a cdp that you can switch it in and out on the fly, that subs it for the very highly regarded DF1704 chip.
If I put one of Professor Keith Johnsons "Reference Recordings" on, which are nearly all HDCD, you can hear the difference straight away, as in depth and the ambience of the venue it was recorded in.

Here is a list of HDCD that are marked, there are nearly just as many that are not marked HDCD, you only know when the HDCD light comes on your cdp or dac.

http://musicbrainz.org/search?query=format%3Ahdcd&type=release&limit=25&method=advanced

Cheers George