The number of remaining downvote points should be visible, otherwise at some point people run into a brick wall that should be avoided by calibrating the rate of downvoting rather than by being unable to make high-certainty downvotes.

It is more of a 'putty' wall. If you run out of downvotes, go to what you previously downvoted and select the least disliked post.

Even better, allow unrestricted downvoting, but recalculate the efficacy of downvotes based on voter's karma and number of downvotes.

Would this depend on the order in which you made your downvotes and the karma you had at the time of voting? If so, I may intermittently go through my past votes and redo them such that my influence is maximised. If not, the karma calculation system would become decidedly non-trivial.