The issue with all the advantages you're pointing out is they're just as likely to be disadvantages when the roles flip around. Just read this:
Say there is a rebel that comes online. He sees that someone proposed a mission. No one else has commented on it. He has to make a decision, accept or reject and say it publicly. And this is a pretty binding decision if you ask me because a reversal is going to bring a huge amount of scrutiny. Afterward others comment (a mixture of opinions) and maybe knowing this he would have changed his mind, but now he can't, or if he does again lots of scrutiny.
You can see the obvious change and hopefully you can see that the logic works basically exactly the same. Whoever's committing first has less information. But why should a rebel then want to commit early. Why not comment on the team, if he feels the need, then decide how to vote later. Maybe it's M2.4, he's not on the proposal, but he has an awkward choice of, vote yes to a mission that he knows has a decent chance of failing, or vote no hoping that the next mission is better. In that situation he won't want to go first, and likely a lot of people won't.
And actually now I think about it, another major advantage is that it gives spies more room to hide co-ordination messages, if they're being forced to justify votes BEFORE missions. One potential big breakthrough resistance can get is putting multiple spies on mission, if both fail, that's a lot of information at once, and the less room spies have to try and hide a co-ordination message, the better. This is why e.g. you'll notice everyone insisting on silence after a mission goes ahead. When you're explaining your votes before making it, then it's much easier to leave a subtle hint, and further, it's basically impossible to pick out from other resistance member posts, whose posts will also look very similar.
It wasn't discussed much last game, because there wasn't much need to discuss it. It's pretty well established in the Resistance meta that it's a bad thing. The advantages to the spies are huge whenever they're voting late, and it's pretty common that a few of them will be every now and again. The advantages to the resistance, I'm still not seeing. I did read your first post, and I really don't see the advantage, because everything I think you're saying is an advantage is just as big of a disadvantage when the roles swap around.
Regarding the last point: There's lots of different things you can do with proposals. Not putting yourself on mission is always an interesting one. That really forces people to think about if they want to vote for you. Another one is proposing a mission when you have multiple spy reads on it together. That can lead to some potentially interesting votes to look back at. It's a minor point, but the more you force people to commit pre-mission, the less able those kinds of game-opening plays work.