It's been a long time since I've looked at the P&P rules, but I could *swear* there's nothing in there about Paladins falling if they ever once perform a chaotic act (unlike evil acts, which *do* trigger immediate fall). It seems to me that a lawfulness index would be perfectly acceptable, even without modifying the quests themselves, if a Paladin fell:
a) For any virtue hit whatsoever
b) For any lawfulness hit that brought their lawfulness index below 12
That is, Paladins must stay Lawful Good, but they don't need to guard their lawfulness as vigilantly as their goodness--as long as they live on-balance lawful lives, they're allowed the occasional slip, but one *bad* act and it's over. A Paladin can conscientiously object to a single order (even one lawfully legitimate) if they think it's evil, but they can't make a habit of disregarding orders (even for the greater good)--they must, in general, live lawful and regimented lives with appropriate respect for authority.