There are so many posts about BungeeCord now - and they all seem to say different things. Here is where I see things at the moment, as the guy who wrote much of the Forge specific code in Bungee and fixing bugs that are appearing.
First, and perhaps most important - you should not be asking whether Sponge will be compatible with Bungee, but whether Forge will be compatible with Bungee.
As it stands, Forge 1.7 and Bungee do work together unless custom packets are involved (but PluginMessage packets should be used instead that anyway). Depending on the changes in the Forge Mod Loader for 1.8, Bungee may stop being compatible, but as we know how the handshake fundamentally works now, and that we have a framework in place, updating to support the new handshake will hopefully not be that difficult. Indeed, if Forge will support Vanilla clients, then Bungee can just act as a vanilla client, and that should “just work”.
To answer specific points:
Bungeecord will need to be remade for either forge or sponge to work.
As I said above, there will probably have to be some work done so that it’ll work with 1.8, yes, but nowhere nearly as much as there was when blood and I worked on it. It certainly won’t need to be “remade” though.
Either way it required a bukkit plugin not a forge mod to work so it will need to be remade
BungeeCord doesn’t need any plugins to work. It might need a core mod on Sponge’s side if you want to support IP passthrough though, using ASM or something like that to inject the relavent code into the relavent packet handler. I may give that a go closer to the time if time permits.
So to clarify further, BungeeCord will have to be remade to support Sponge?
Remade is the definately wrong word. Modified. And, to be honest, really the question you should be asking is “what do we need to do with Bungee to support Forge/FML 1.8?”
If you have any more questions about BungeeCord and Forge, then feel free to ask me.