Well good luck and let us know how it went. Looks like you may want to talk to the manager at the Apple store to get this escalated. Looks like reading the link posted above, the OP of that thread did manage to get a refund. It's worth trying. And if they don't budge, then you can just sell the 6 Plus for a profit and get a 6 instead.
Plus, you mentioned they never said anything about the replacement being not returnable when you did get it replaced. So I'm pretty sure you can make a valid point that you were never told about this. For me, they told me about it so that is why i decided not to replace the unit. I might have to keep checking the store for the 6 Plus availability and if they do have it available I will just go with the return/exchange route instead.
Plus, you mentioned they never said anything about the replacement being not returnable when you did get it replaced. So I'm pretty sure you can make a valid point that you were never told about this. For me, they told me about it so that is why i decided not to replace the unit. I might have to keep checking the store for the 6 Plus availability and if they do have it available I will just go with the return/exchange route instead.