Compulsory comment before the flag can be applied

I don't agree with this, because I don't think we should incur any kind of cost (cognitive or otherwise) to downvote obvious spam. You do however give a good point that people should be able to reply to a flag directly. Perhaps they could set it up so that a flag can have it's own comment thread automatically if people object to it.

I think @dantheman's latest post about Google-style ranking could solve these problems. If someone is abusing their voting power, you can give them a negative weight vote on their profile. This should discourage abusive behaviour of any kind including downvotes.