> Is there a word count/rewards ratio where you deem something is over-rewarded? Could it be that objective?

It is very subjective.

> For instance, self-voting is tolerated, but if I gave this comment a full upvote on my POB account, I "feel" that would be malicious. Would I be correct in writing that observation?

In my opinion?  Yes.  I am personally not a fan of self voting comments.  In some rare cases I may do so with a small vote to bring it to the top of the comments if it is something critical.  But in almost all cases self voting comments is just farming.

> Are there types of malicious upvoting?

All the conspiracy posts that take half of trending that get massive votes for pseudo science.  All the whales that voted 5-20 vote posts that had no rewards to maximize their curation with little effort (pre-hardfork 25).  There are a few whales dropping $15-40 votes without regard to quality or even content and they stack on top of each other to take posts near $100 when they normally only take a few cents.  Just look at trending, you will see the same 5-6 whales controlling it with no regard to quality. 

I could go on, but I will struggle to find many cases of this elusive "malicious downvoting" that we are trying to stop by making laws on how to vote with their stake.