To be clear, my information is out of date because I don't do this kind of work anymore, but my understanding is that it's largely the same now.
1. I know because I sold to my list, and the people who signed up from intrusive signup popups were just as likely to buy my products as anyone else, and their virality was the ~same as other sources (ie. the rate at which they shared my stuff). I guess it's possible that an army of bots was sharing my posts and buying my products, but..?
2. Yes, bounce rate is a basic thing almost everyone monitors, and yes, intrusive popups increase bounce rate. But less than you might imagine, and not nearly enough to turn the tide of increased signups.
Like I said, I find it counter intuitive myself, and have no real explanation, just made up stories about it. All I know is that what I consider to be intrusive signup processes have had a life altering effect on my bank account.