This should be quantified ahead of time. No posts in 3 months, 6 months, 1 year? Inactive as of the start of the competition? Or what if an inactive account wakes up during the competition?

This feels too restrictive to me. I would have opted for a lower number, say 20 earned merits, but that's just me.

I appreciate I am heavily biased here given I have previously won the same category twice, but I wouldn't take this approach. You could easily end up in the situation where someone is deemed to have won, but someone else would have won the "popular vote", by getting more votes which were each worth less. I can already picture the arguments which could arise, particularly if there will be prizes involved. It is also disincentivizes people from voting for the previous winner, since their vote will be less valuable than if they voted for someone else. I would either go down the route of excluding previous winners or doing nothing at all.