Wikipedia is not the best source... Check out this analysis (in czech) with a map of the wreckage (proving that it was spread over large area)

http://technet.idnes.cz/serial-teroristicky-utok-nad-cssr-pr...

It also says that FDR and CVR were analysed in Amsterdam by dutch and american experts. So they are probably pretty sure in which altitude were they flying.

It also includes chemical composition of the bomb that brought it down.