To be clear, activity date is what matters - posting date is completely irrelevant to this promotion unless an activity posts after March 2012 (or whenever they stop checking for activity).

I don't see any upside to emailing them in advance. They're just going to quote the T&C's and since you're not actually out anything, there's nowhere to go with it.

If, on the other hand, you complete an activity in advance of the deadline believing it to qualify and it doesn't, you can show a good faith effort and have proof that the activity was completed in advance of the stated deadline.

It's not reasonable for you or anyone else to know how vendors post activity date, and a reasonable person would tend to believe that activity date would be the date that you completed the activity.