This post is in the wrong location. I'm moving it after I finish this response. PLEASE read the posting guidelines before posting, this should really be multiple posts in different places.
1) Many fonts have Cyrillic letters. Try unifont perhaps. As it is, if you can make BDF format fonts, there is bdfconv tool in the rockbox sources for converting them to the Rockbox format.
2) Yes, it's an annoying bug. It also happens on iPods. It is NOT an easy fix, and will take time. Deal with it.
3) How are you coming up with 100x100? What evidence do you have?
Also, backdrop images must be 24-bit. 256 color will not work. Do you still get an error message if you try it with a backdrop image that actually follows the rules? If so, what is the message?
4) WAD files are the actual game files. Read the PluginDoom wiki page.