Alice votes on Carol and Bob downvotes Alice's upvote but Carol still gets the author rewards that Alice sent her.

I think the solution is the exact opposite - Carol gets nothing from Alice's vote but Alice still gets her curation reward.

Why? Not because of Alice (she is doomed anyway). That's because of Dan. Now that Carol's post that deserves $10 stands at zero, the good curator Dan is expected to upvote it, isn't he?

No, he is not. Maybe inside your head. Unfortunately, under the current rules, Dan is disincentivised to do so as he loses part of the curation rewards due to upvoting a post that was DVed. The same applies to upvoting any post that can get DVed in future. Carol is damaged by Bob not by chasing Alice away (she would upvote for less if she delegated negative HP to Bob in the fictional world anyway), but by chasing away her other - current or potential - curators.

The current system sucks because a downovote has a huge footprint, something that upvotes do not have at all.

Happy 3-month anniversary everyone and talk to your favorite witness now.