Well, I've no idea how to do this.
It definitely can't be done in any of the drivers, if at all it would require changes in the MESS system itself (and that's nothing I would like to touch too much).

But before that, one question would be important:
does this (i.e. needing a double-click instead of a single click) only happen for MESS, or is it also happening for other Windows programs on the tablet?
I still believe that it's rather an OS (Tablet-Windows) issue than a MESS problem.

And an other idea:
In MESS there are lots of settings which can be changed for MESS itself and/or every single module - see 'Options' menu in MESS, 'Properties' in the module's context menu (right-click in the list), or 'File > Show Tab Menu ...' when the module has already been started.
There are many settings for keyboard, mouse, screen etc ... - have you ever tried to modify any of these options on the tablet?
(it doesn't make sense for me to try this, because I don't have a tablet for testing).

Regards,
Franz