That is a good point with the slider, I don't know why it is withheld.
My guess would be that when this feature was designed or when the limit was introduced, accounts that had under 500SP would have too small of an upvote anyway, so making their votes even smaller might have been viewed as pointless and just allowing them to cast just too many virtually meaningless votes. The thing is now when the maximum voting power went up, people that don't have access to the slider can't really revert to the old setting and are forced to either be stingy with their votes (which I hate) or to allow their voting power to drop to really low levels. I personally experienced that last week while I was not watching my votes, so my ability to upvote with even of small value had pretty much disappeared because I consistently overshot the 10 upvotes per day mark. So I was stuck casting low value votes anyway without the ability to regulate that properly and spread my SP a bit more evenly without being stingy with it.
And on you other point, a one word comment with a 50 dollar reward is bound to look fishy and uncool regardless of the way the value got there - selfvote or not.
I know the issue as some of my family are here awaiting a slider and they are in the same boat that would like to vote more but feel they can't. Perhaps it will be cleared up in the future.
Yes, there are very few reasons for a comment to get that kind of support.