All suggestions are welcome, but I am limited by my free time (and stuck at the moment because my laptop motherboard died on me

, waiting for repair or other non-expensive solution and using other people's machines for now).
If cbr files don't show thumbnails it could be because the rar archive is either password-protected, uses "solid" compression or has split volumes (see FAQ).
No thumbs for Compressed Folders can also be sign there's something wrong with those zips.
Check'em.
Didn't Adobe (or others) fixed pdf thumbnails issues on x64?
Unrelated, from
previous posts:
I got bored so I did some exercise in minimalistic C.
A COM wrapper dll to get thumbnail as HBITMAP:
http://www.autohotkey.net/~T800/CBX/cbxstub20.zip
(x86 & x64 dll+source)
When I get to it, I'll make next CBXShell version that will export function to get thumbnail directly (maybe; so many plans, so little time).