Nice Combo evaluating tool available!

True. Two positives becomes a negative in this case.

Solution:
* 1 real-time AV
* several on-demand only AV

I always scan with several on-demand AV (for big files) or upload to VirusTotal (for small files) before I execute any file.

A nice and handy tool called File Submitter which can submit the suspicious file to various sources and get the scan results.

 

Yes, it works with CastleCops MIRT team together, so this site is genuine. However it doesn't imply that the test result is of high quality.

I have doubts of the following:
-- the cleanness of the database
--- how many corrupt samples?
--- how many false-positive samples?

The result alone can be very misleading since it doesn't take false positives into account. A scanner which flag every sample will score 100% easily.