Trouble with adding second xenon joining a network

You should not unclaim your devices, doing that will keep your mesh networks in limbo.
Currently you cannot disconnect a node from a mesh any other way than by assigning it to a new network of yours.
This quirk should be fixed, but if you don't want to pass your device on to anybody else, just don't unclaim it. You can re-claim your own devices over and over without ever unclaiming it.
Just claim it and assign it to a new network, this should then remove it from your networks. But you can't fully remove a device from your network while it is not yours.
Also you should first remove all "client" nodes before you remove the gateway from the mesh.

That would currently be setting up a HA (high availability) network with is not yet supported.

That might appear to be the case but in most cases we came across it's not that it wouldn't work but that the instructions are not very clear and the users usually miss some of the nuances of that less than clear set of instructions.

A similar question is addressed in this post