The issues you are touching upon relate to Identity and the agreed social contract of playing here.
1, Alliances: when people sit down to play a board game at a friend's house and they each agree to play two countries, that is not an alliance that is a game variant which all parties have agreed to. When a multiple is created and by deception you are inflicting it on the other players in violation of the social contract of one person one country.
2. As for multiple accounts playing in different games, in this community you are allowed to hide your real identity behind various nicknames or handles such as MonkeyBrain91201 or whatever. However, herein begins the ethics of this community in that they want you to have only ONE identity regardless of what you may call it. Here, there is a point system that depends on its function there being only one account for the player and there is no option to elect not to participate in that point system.
Furthermore, while people here have strong attachments to their non-real world identity, they also use it to establish a reputation for that new identity either intentionally: you will see people say look at my games I keep my alliances, or unintentionally such as look at his posts in the forum he is an idiot.
For some it may seem like a contradiction: to allow people to hide behind identities as choosen while at the sametime allowing them only ONE identity. But as you parents may have told you, life is not fair, it is not consistent and the rules are not documented in detail for every situation...as opposed to Diplomacy which is.
For some, and for many of the youth here, it might be good and reasonable to have one identity for a while and then after an absence come back with a new identity so that the destruction that they may have caused to themselves can be more easily relegated to the past and left behind. However, that is not something that has been codified here but may come down the road either as a natural issue of being unable to enforce it, or as a matter of a policy to support the alien concepts in life of restitution/reconciliation and redemption.