Generally (especially due to the Coronavirus isolation) some groups of friends will all decide to try out diplomacy and start playing some games together. This allows for friend groups to remain in touch and have fun together.
You can usually recognize these games with (as you said) the signup times for players, starting points, and the same games. Also if you look in the games, you'll usually see many unorthodox moves (since they're probably beginner players. For example (though I don't have a game attached): Moscow to Livonia Spring 1901 Classic (happened to me in person), then holds in Autumn 1901.
I even do this. My school (though I will not disclose it) has a Diplomacy club (like thing) and we'll usually play games on vdiplomacy since some people can't make it to the in-person meetings.
However, it's very important to not have players in games where there are other people (not people that you know). This breaks the No Metagaming rule (quoted from rules) "You cannot play a public game with players that you know outside of the site. In doing so, you create an unfair environment for other players..." (Rule 2)
Whenever I play a game with my friends, it is that. ONLY with my friends. If others want to play games with their friends, then they should make it password protected and only play with the people they know (or only agree publicly to make agreements via the game chats). Inviting people from the public breaks the Metagaming rule and creates unfair advantages and alliances.
Sorry for the long response/rant, it's just that I have experience in both sides of this argument.
Also please don't dox me (from games with friends)
Oh and addressing #4, that usually happens due to a single/multiple players become disinterested and going into Civil Disorder, thereby creating an unfair board (where one person dominates). This is actually happening with most of my current games.