It's as simple as what they tell: they're too stupid to ignore the 3rd
mouse button and instead exit the game.

That's bad ! Is there a way to deactivate the build-in mousedriver from dosbox ?

No, but besides that the game doesn't work without a mouse anyways.
Can you hex-edit files?

yes

offset 0xbb8e: 0x83 0x3e 0x62 0x91 0x02 ->0x83 0x3e 0x62 0x91 0x03
that is change 0xbb92 from 0x02 to 0x03

Many thanks!

Is it playable? By times the mouse did behave erratic, but might have been my build as well...

yes, its playable. On the beginning of the game, you must type your name.