After installing SP1 and SP2 for FSX, I attempted to re-install all my DreamTeam sceneries. First, I tried it using the files I'd saved during purchase
There wasn't any need to save the products during purchase, because all our installers are always freely available and, since the time you purchased them, chances are we updated them so, you would have to download the new installers anyway.
did not work (got a message saying SP1 and 2 required)
If the installers said you didn't had SP2 (there's no message saying SP1+SP2 is required, the installer only checks for SP2 or Acceleration, which of course can't be installed without SP1 first so, it's the same) it means you didn't had SP2 or Acceleration OR you somehow installed them in the wrong way and something didn't work in the FSX installation.
The installer checks the actual FSX.EXE version so, it can't be mistaken, if you installed SP2 and your FSX.EXE is not the SP2 version, something went wrong and you FSX install is not ok.
Then, tried re-downloading the Virtualli and new file for PHNL and KJFK. Neither one would install; same error message
Well, of course it would't change with the latest installers, if your SP2 installation didn't completed correctly, it's nothing the new installers can fix...
I have had absolutely no problems installing sceneries and aircraft from other vendors
That doesn't mean much, not all installers check if you have SP2 or Acceleration installed, ours will, because without them correctly installed, nothing will work.
You can see what was wrong, by right-clicking on your FSX.EXE, select "Properties" then "Details", and have a look at the version number, it's a number beginning with 10.x. and should end with 61637 (Acceleration) or 61472 (SP2). If you see another number, it's not the SP2 or the Acceleration version.