As a freelancer, you can simply refuse the monitoring feature. I would never accept a project where the client wanted screen shots. I use Google Docs so my activity is tracked there pretty accurately as it is, but I think most freelancers aren't taking the monitored projects.
I'm not sure what you're asking about the 'client questions', but you are able to ask questions before accepting an offer or even making an offer. I have had clients make me an offer before any dialogue took place, but that's rather rare, and I always clarify my work process with clients before accepting offers.
Upwork is not my primary source of income - I have a private resume writing business that I run - but I use Upwork because it's brought me quite a bit of business without having to do any marketing (at the cost of 20% of my earnings). I'm happy to give them 20% based on the volume of business that comes my way there.
I primarily have used fixed price projects as well, but mainly because time tracking in Upwork is a bit cumbersome. You need to track in 10 minute increments, and my workflow often finds me in documents multiple times per day, but for short periods of time. That's not conducive to time tracking, so I try to set fixed price agreements.
I've been pleased with my experience on Upwork, but I know part of that is because I've had the benefit of being one of the higher ranked writers there for a few years and I have a larger stream of income from other sources, so I have the luxury of turning down quite a bit of work there if it doesn't meet my preferred criteria.