In my view absusive downvotes are mostly a problem because there are too few downvotes. As a result, when downvotes do happen they have outsize impact both financial and emotional. The solution to this is not to nerf downvotes, it's actually to make them better. Your suggestion of anonymous downvotes is one such way, but there are probably others. I would actually prefer that all votes (both up and down) be anonymous, as is the case on most centralized sites. Focus would then shift from who is voting and whether that vote is "legitimate" to whether the net payout is appropriate (and if someone who has not voted disagrees, they can vote up or down to express that). That's the right focus. 

People should be able to "like" content non-anonymously, a social operation that doesn't affect payouts.

More directly to the point here in your post, I think it would be possible to take the downvote curation reward penalty only from those upvotes chronologically before the downvote, so upvotes to counter the downvote wouldn't be penalized.