Downvotes spread more widely would indeed help with some cases, since then getting downvotes would not be so devastating to rewards (most content would have some so the zero-sum aspect would mean it would tend to balance out some).
Though it still wouldn't help with these "downvoted to zero" situations. Again, people have to accept that in a voted-reward system, highly controversial content with a lot of disagreement won't do well on rewards. We don't (for the most part, though front ends do hard block child porn, etc.) censor it, but we won't reward it either.
Layer two where a narrower subcommunity (i.e. with less disagreement on some things) can decide differently about what it wants to reward is probably the best solution.