getting this message when trying to install KJFK, KFLL and KLAS ...
The message doesn't normally happens, unless you have a problem with your XML files, caused by having hand edited, or by other products faulty installers, the installer is just saying it cannot parse the file because it was ALREADY corrupted BEFORE running it.
You should either try to fix it the error, or just remove it, and the installer will create a new correct one.
too many bugs trying to use and install these sceneries
There are no bugs whatsoever installing the sceneries, and you can't just pretend our installer could fix any possible already existing problem in your files.