I'm at a bit of a loss really. It's not like when you do this on PCs that can have thousands of different hardware configs. Mini9's should all be the same. All I can tell you is that with the default BIOS on A05 I haven't had a problem with the retail SL DVD restored to a USB drive and using the method I described.
It would be a bit naughty, but you could borrow a copy of SL to do the install, then if it works OK, buy it.
Thinking about it, aren't there a few different version of the 10.5 DVD? Some that are 10.5.0 then others at 10.5.2, 10.5.6 etc? It could be there is a problem with the installer/Mini 9 and the version you have?
---------- Post added at 08:58 PM ---------- Previous post was at 08:58 PM ----------
One last thought, have you got anything in the WWAN? Pull it out for the purpose of the intsall if so.