It was a deliberate choice to require only the left button to do this: on my preferred type of trackball left-and-right-clicking is annoyingly unwieldy, and my Minesweeper solution time has dropped by about 30% just from this one UI change. (The trackballs are fantastic for most things, but the traditional Minesweeper UI seems to be the one common thing they're not good at :-)

Mouse handling is done centrally in my puzzle framework, and as you observe the pressing down of one mouse button fabricates a button-up event for any previously pressed one. This is usually a good thing, though I concede that it isn't ideal for Mines. Feel free to submit a patch :-) The relevant function is midend_process_key() in midend.c. All it'd take would be a bit of configurability in that function and a set of flag bits provided by the individual game.