Unfortunately it would basically be an ongoing DDoS on the court system to try to get declaratory relief for existing garbage patents and the never ending stream if them created every day.
Maybe. I’m not sure anyone has tried it. It wouldn’t work against a party like Intellectual Ventures but in theory you might be able to take out a nascent troll with a smaller war chest, and if you win they might have a tough time selling off their portfolio.
2. You can't sue a parent troll for patent infringement, since they don't have any technology business. They are only "companies" on paper.