Two things make this not as gameable as it may first seem:
Piling on late is just tipping late. It still counts, but later tippers get the smallest share of the credit, so waiting has a price.
Nudging a rival under the line means paying for a downvote, and that money goes into the rewards pool you're competing for. Plus it gives everyone else still in the running a tiny raise, not just you.
On "most time in the top half", I like it in theory, but we'd have to track how every post's score moves all week, adding complexity, and like you said, it also has its own quirks.
Two things make this not as gameable as it may first seem:
On "most time in the top half", I like it in theory, but we'd have to track how every post's score moves all week, adding complexity, and like you said, it also has its own quirks.