I can't open certain "non standard" excel files in Mac Numbers. Can LO offer me a way around this snag? How hard is it to install Linux if you are a basic, non-techy user?

OL for Mac - Difficulty level for non-tech savvy user?

Just install LibreOffice for Mac OSX - no need to install Linux to be able to run LibreOffice. Both PPC as well as Intel are supported by LibreOffice (from and including Mac OSX 10.4 upwards)

(and installing linux is dead simple nowadays, the times where one needed to fiddle with the commandline to install are gone, unless you’re trying to install on “gamer’s” notebooks - those might need hardware tweaks - just run one of the linux live-ISOs and check whether your hardware is supported)