Adding UI elements is very "expensive" in the sense that it's something to which every member constantly has to devote some brainspace. So I can't see adding any kind of split merit score or other UI element, since that'd be a permanent high cost as a result of a one-time thing in the past.

I also can't see decaying non-airdropped merit scores.

I could see decaying airdropped merit, though I'm not convinced of it currently. If it was done, it may be best to do something like this:
 - Your current activity score as of is recorded, which I'll call EPOCH_ACTIVITY.
 - Your merit score is defined as max(0, AIRDROPPED_MERIT-(CURRENT_ACTIVITY-EPOCH_ACTIVITY)/7) + EARNED_MERIT.
 
So you'd already have had a lot of time to earn merit, and you'd get an additional 7 "activity-days" per point of merit to get back even more. Inactive users wouldn't be demoted.