Reappearing in “newest” is like winning the lottery. Both never happened to me. The spam can be solved by adding a way for players to block games/developers so that they no longer see them. And if the players don’t like the new section, they can continue going to https://itch.io/games/newest
You need to do a major update for that. And what is and what is not a major update is unknown to me. By observation it is not more often than once a month. And as a speculation, it might be account bound. You have many projects. So maybe most of your updates are considered minor, since you already published a whole new game that month. It might be that staff has to approve them, but I doubt that they have time for that. So maybe it really is a lottery, who knows.
This might be so. But this would also be quite unfair to the developers that simply update their games regularly without logging about it - if this were the only way to have an update be considered a major update. Even more so as devlogs have their own place to shine and be found, regardless if there is any associated refreshment of the recent status.