Agreed. CPO is really just nomenclature, and only applies under specific conditions. It was CPO if purchased through Nissan (dealer).
By his own admission OP purchased through another manufacturer dealer, and understood he forfeit the warranty. CPO without a CPO warranty is not CPO.
Just because it could have been CPO under different circumstances is meaningless.
Furthermore, I tend to agree with Lehto that the whole CPO thing is kinda ridiculous (I'm paraphrasing). They're trying to convince the consumer this is something like a third category between new and used, but really it's just used. It feels a bit like calling your second mortgage a "line of credit." It's just a mortgage. And CPO is just a used car with fancy letters -- especially when any warranty is forfeit.
The fact that Nissan is willing to replace the frame under the circumstances is impressive and stand-up.
A "CPO" certification is merely a means to sell an extended warranty, and make the purchase of a used vehicle more palatable.
In other words, make it seem more refurbished than used up.
The saga of the OP is largely fluid.
When one says "If its advertised CPO that means it was," does not mean that it is. He tends to think that that means something.
It doesn't mean squat if you don't purchase the CPO warranty.
So, he just needs to stop polluting the argument with it. It is not germane to the situation.
In regards to the frame, It's obvious that Nissan screwed up.
They stepped up and fulfilled their obligation. Good for them.
God knows they can't afford any more bad PR.