Just a question - why did you buy a N900 if you don't have internet connectivity? It's an internet tablet.
How did you download the .debs you want to install? If you can download debs, surely you've got an internet.

Installing .debs and their dependencies manually is an intricate process, whether on the N900 or any other debian-based PC.

Anyway, if you're manually modifying files in /lib or /usr/lib etc., please run ldconfig afterwards. You can find missing dependencies using the ldd command (if that's installed on the n900 - I've not checked).
For example, ldd /usr/bin/gamebinary

But really - you will make your life a whole lot easier if you get your N900 on the internet. Even if you use 3G, a lot of the apps aren't very big, so shouldn't hurt your monthly download quota that much.