Anti-trust regulation is basically dead in the US at this point. And you can bet your top dollar that Zuck/Cook/Bezos/Pichai are only going to continue trying to cripple it more. So I don't think such lawsuits are a credible threat or deterrent. Just some minimal ongoing cost of doing business.
A lot of moderation trouble and not really a direct competitor to one of their products in the way WhatsApp threatened Messenger. Much higher risk of users abandoning the platform too due to weaker network effects.
Reddit and similar websites are definitely a long-term threat to FB. A lot of the people I know use FB just for the messenger and almost all of the time they used to spend in FB has moved to reddit.
I doubt it. Zuck needs to be subtle with any changes. Just serve the ads from FB, let everything run the same for a couple of years. Then an option to login with FB creds. Then make FB login mandatory for new signups after a couple more years.