Nice that you're finally talking seriously about this problem and you intend to act but I think it's simply to complicated. And when things get complicated there are more ways in which the system gets abused and cheaters bypass it.

Rather than implementing all this, why not try simpler solutions as hilariousetc, suchmoon or Loycev have said.

- make the topics bumpable once in an interval, not once a day, this sounds too much, but maybe 8/6/4 hours?
- make so that only users who have earned 1 merit (in the last 30 days!!, to deny merit farmers) can bump a topic

Or we could go to an extreme solution!
- bump the topic only when a merit has been awarded in that topic