I have this card, and like it. In VMC, it gets OTA ATSC in HD, plus SD from the cable on the other input.
As for Clear QAM, VMC does not support it, but the Aver Combo card supposedly does via their own "media center" software.
So, more direct answers to your questions:
1) Yes, you should get HD OTA in VMC with this card.
2) You can feed the set-top box output to the digital in of the AverTV card, and you can tune in the Clear QAM stations; however, they will be downconverted to SD. This is a Vista MC limitation, rather than an AverTV card limitation. No idea if MS will open up this capability.
3) No idea...
4) Again, no idea...
A.L.a.E.o.t.U.S., as proven 3/21 - never forget.
Knowledge isn't Truth; it's just mindless agreement.