The problem for sites arises if different ISPs give them different prefix lengths --- that will really mess things up at the subnet level.
No, you just zero out the prefix to the length that applies for that connection. As long as your subnetting fits within the longer prefix, you're still fine. If not, then you need a shorter prefix...
Sure. There's plenty of space to do that. But don't forget the other big problem of 8+8 or GSE -- the transport layer has to be fixed up too, unless you can arrange for all goops to have the same checksum ;-)
Not a problem if, as you suggested, the host can be forged^Wtrained into always expecting the most significant bits to be zero and if the CE is always stepping on the inbound bits.
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