If they can't and their flank was in the air then I see no reason why they should not be penalised for it. Certainly if one unit is attacked by two units with a staggered attack (i.e. two charges in successive bounds) then initially the unit would engage the primary aggressor (i.e. turn to face the charge) and then if this left the units flank open then it would be charged by a second unit. Can't see that this is a problem (or unhistorical).
I agree that the possible solutions all suffer from tedious complexity.
OTOH, the potential for conforming shifts to strip away flank protection (i.e., eliminate a ZOI) and the risk of rewarding/encouraging angled approaches to prompt radical shifts in the enemy battle line smacks of the geometry wars of DBx and the single-element pulls of that game that often annoyed players.
It also seems to me that swifter troops (e.g., mounted and lights) are more susceptible to this ploy because they have greater movement to cause wilder adjustments. This would be a big departure from older sets that forced the player initiating combat to conform to enemy.
Accordingly, if there is a way to temper the results of shifting/conforming -- a rule that seems to be aimed more at table tidiness (not to belittle that goal) rather than historical combat simulation -- that is worth exploring.
Spike