It should. Both when downloading .deb directly (open button, not save) and also there should be 'install from file' somewhere in application manager menu. Could you verify that it works? Just click the download link on official site and select open button not save.
Yes, all tablets are good enough. MI3 is playable even on 770.
real backspace may be needed in some games, maybe ctrl backspace is free, '.' is remapped to some arrow key since N800/770 have no keyboard. As for keybindings - they need to be selected carefully, each game engine has its own keymappings and even ctrl and shift combinations are used in some games.
Also maybe you will be missing ESC in other software too, another way is to change N900 native keymapping to add tab, esc, pipe, page up/down, braces etc. For N810 this is in /usr/share/X11/xkb/symbols/nokia_vndr/rx-44, maybe there is rx-51 too?
There is slowly going project inside ScummVM to provide generic keymapper and virtual keyboard, it is stil not ready though. Many game engines have keys hardcoded so even with keymapper available most engines needs to be ported to use keymapper instead of raw key scancodes.