It seems to me that Harriet has the rights to RJ's unpublished notes but Tor owns the rights to all the published stories. Thus it was in Harriet's power to commission another writer to finish the series, but honestly, the statement isn't as conclusive regarding new stories in the WoT universe. I still go back to what I said earlier: We don't know the extent of the rights and the various limitations because A) it is possible that Brandon is not aware of the full scope of rights and the specific delineations within each set and B) you can't copyright the title of a book, short phrases and other elements which may form the backdrop of the series - going by standard copyright law, it makes more sense that Tor owns the rights to the books in certain media in a limited buyout and after this agreement's term runs it's course or to expand the media, they may need to seek Harriet's approval.

I don't know - I think it's less trustworthy to trust a commentator that is not a party to an agreement, which is essentially B. Sanderson's position, versus a more detailed statement from the copyright owners such as Harriet and Tor.

So to me, this whole question is more about what RJ signed over to Tor in the contract and less so whether they would proceed without Harriet's blessing; doing the latter seems more of a matter of good faith than a necessity, but we really don't know without having the contracts to review.