Again back to the analogy, I have wronged person Y by selling something which was not mine. If person X decides ok $5000 is a fair price, person X cannot come after me for the $5000, I have no relationship with person X. Person X is still the rightful owner of the car. Person Y would have to retrieve the money and enter into a separate deal with person X.

OK in Novell's case there APA forms a relationship with SCO, but since the APA does not permit selling of new licenses or substantial changes to existing license, Novell doesn't have that contractual relationship with SCO.

Even it somehow they can, it would seem dangerous to me, as it is effectively setting a precedent that SCO can do such activities provided they send on the royalties. SCO could do the same in the future (on very different terms) and Novell could be in an estoppel position  due to the acceptance this time for SCO doing so.

Not to mention Novell would be particularly foolish (in my view) to get involved in contracts for which it has no knowledge of the breadth or depth of rights issued. What if the rights issues go beyond the bounds of even Novell's rights to third party code?

Regarding SCO being estopped in their defence, of course they are. The defence will be as you say that they believed they owned the rights and it's nothing to do with Novell.  If the judge finds that Novell do own the rights, it's no different Novell don't take over ownership of the contracts, the contracts are voided.