In Ex2 I would model this (exploiting someone's customs) by:
(i) creating a Charm which imposes a new Intimacy that is culturally-appopriate, or else reinforces an existing Intimacy that is culturally shared;
(ii) creating a Charm which compels behaviour aligned with an existing Intimacy.
Skillful Imposition Method method is narrower, shorter-term and has a smaller narrative impact than the above. It's shorter-term because you don't create or reinforce an Intimacy. The effect works in the here-and-now: for
this social action, your counterpart
is treated as having a minor Intimacy. It has a smaller narrative impact because you're not compelling any behaviour; SIM simply gives you a social combat advantage.
Based on my Ex2-calibrated intuition, this Charm feels too narrow and too expensive to use. If Ex3 social Charms in general are more specific (and this may not be a bad thing!), the Charm's scope may be fine. I still have trouble swallowing a 6m cost that does little more than give you an Intimacy (a minor Intimacy, even) to exploit. That feels like paying a lot for a pretty basic effect that is
also niche.
Tentative comments on balance aside, the general feel of the Charm (encouraging clever tactical use of an opponent's preconceptions, beliefs, etc.)
is cool. Social interaction is a lot more interesting when you're encouraged to scout and take advantage of an opponent's mental terrain to get your way than it is when you can simply impose your will by Lawgiver fiat.