apologies for this complete newbie question but are PC CD-/DVD-/BD-ROMs even able to read the files on an AmigaOS CD?
Yes. I use a CD to transfer files between my PC and Amiga all the time. Make it re-writeable and you can dump files downloaded on the PC onto it then read them back on the Amiga, that's how I get everything onto my Miggy without having to waste a whole stack of 'em. I remember back in the mid '90s when PC CD's started becoming more common, it was always fun to pop them into my Amiga and see what sort of pictures/video files/sound files I could rip off them.
Only issue you might have is with long filenames. If I can remember to do it tonight I'll throw my 3.9 CD into my PC and confirm that everything is readable, but just off-hand I'd say 99% certainly.