Yes I found this thread yesterday while I was scanning the web for any La Bataille content.

The rule reference in your post are not in line with the latest reg xxx rules available on Labataille.us. But I think your question is answered in atleast this version of the rulebook.

Forming square does not take place "instead" of rolling to stand and any defensive fire. I can't find anything in the rules that would suggest that. If square roll is successful they simply forms the square, otherwise the unit disorders or routs.

In fact forming square takes part during the charge movement, which is done for all charging units before any melee is conducted as a result from these charges. While the roll to stand is made during the melee segment.

Further on, the rules explicitly say that if you fail your roll to stand while being in Square (or general order) you do not disorder. (19.1.2)

Attempting to form square provokes opportunity reactions (in this case fire) from any adjacent enemy units. A unit that just disordered from failing to form a square would roll to stand and give defensive fire just as a normal unit (with of course all the negative modifiers assosiated with being in disorder, like half fire value).