I feel that 3 would not be improbable as we get to a certain rate of infections (my guess would be around 10000-20000 cases in a nation, based on how much mental pressure that number would put on the population and the government) that milder measures didn't manage to slow down enough. They'd of course try to be as less threatening as possible about its enforcement, but in Italy some small areas where the virus showed up first are already in quarantine (though they are talking about opening them up again now, since the virus is clearly out of them as well). If the economic damage keeps piling up at this rate they might consider end up copying what China did to stop it more quickly, and after that many cases I guess that people would accept it less begrudgingly.
I'd also say a fourth one is possible, the milder measures to contain it growing in efficacy as governments get more examples on the consequences of a determined reaction. In Italy my perception is that politicians acted in the first period in a very disorganised way, everyone tried to show they were doing something to gain consensus, nobody wanted to do anything that would cause too much economic damage. As people's reaction made clear that economic damage would ensue anyway, they started coordinating more. As more nations try stuff politicians might learn from their successes and failures, and would be more justified in doing the stuff that worked somewhere else even if it would be unpopular otherwise. And the advice they receive would grow more accurate as well. From what the government is saying here, we should understand during this week how much mild countermeasures work.
The effectiveness of it would still likely be influenced by how people perceive the situation, and that also seems to be getting more accurate. In Italy we had medias making everyone panic for the first cases, sponsors made them calm down, so half people are panicking and half are thinking it wasn't so serious after all and mocking those who did (I'm kinda guilty as charged with this as my earliest reaction). I'd have reasonable hopes that other nations would get the hang on what to say to the population a bit quicker, seeing how we... didn't.
This might still be not enough than the loss of efficacy caused by how much the infection has spread for early mistakes, but it still might slow it down.
5, virus being selected to become less severe is looking kinda good so far, given the news on the two strains and its interactions with quarantine and countermeasures. From the news I've read it seems that the more aggressive form was responsible for 70% of cases in the first stages, but dropped quickly due to the human intervention that selected the less aggressive one. Given that this is very recent news I wouldn't put complete faith in it, but it's the best one I saw so far.
Even if 2. usually takes a year, there aren't really precedents of a virus doing so much economic damage to so many nations all at the same time. Wouldn't this concentrate an unprecedented amount of resources on finding cures and vaccines, greatly decreasing the time needed? I get that ten people can't finish a project in a tenth of the time, but I'd expect it would at least be quicker than usual?