"It does look to me that a unit in line on a road running between town sections effectively blocks the enemy from deploying in any surrounding town section."
Why/how would it prevent that? Any unit, friend or foe, could enter an unoccupied town section from any number of angles, not just from the road.
" I also cannot see any way this unit could be charged legally by any enemy unit."
It certainly couldn't be charged by a Deployed unit in the adjacent town. Trying to form up a battalion of 600 men on a dirt street, in order to charge an enemy already standing in that street... that just seems impossible to me.
As for whether it could be charged by anybody from anywhere... that depends on whether any bit of it is "poking out" from behind or between town bases. If we're using really small figure bases and/or big town bases, I can indeed imagine that it might have "hidden" in the passageway between the town bases. If that is happening in one's game, then I suggest fixing it by simply pushing the town bases together and not running the road between them.
"I also suspect it effectively could not be fired on (one enemy base might be able to, but that is unlikely to be effective)."
If there are enemies in the immediately-adjacent town bases, then certainly they could fire upon him.
"It could arise that the town would be surrounded by terrain that would make it impossible for such a unit to leave based on movement restrictions through rough ground, or by enemy units blocking their escape."
That is indeed a possibility, if you marched your battalion into a very restricted town, and the enemy bottled you in. My understanding of towns is that they tended to be little magnets that drew units in, but rarely let them go. Lasalle is probably much too liberal, allowing you to just pick up and leave in a march column, whenever you want.
"In march, the formation a unit must adopt when leaving a town base, they can move 6 BW, however when in march formation the depth of the unit is thus 6 BW, meaning a large Austrian, or any large unit for that matter, can only ever leave a town base exactly perpendicular to one of it's sides, and have the rear edge of it's last unit be flush with the edge of the town."
That would only be the case if your bases were perfect squares. Most people use a base that is somewhat less deep than it is wide, and thus a six-base March column is actually only about 4BW long.