1. Estoppel is only going to get you a very limited set of rights. It's highly unlikely a judge is going to find you have the same rights as the open source license, unless you were actually relying on all of those rights. You certainly would not be able to sublicense those rights further, except to those you had already. This is not the same as Novus's ability to "stop licensing the software to new parties", it means you would not be able to give others the rights you had gotten through estoppel.
In effect, estoppel mainly going to protect you from damages, not give you the right to use it as open source.
2. It certainly depends, but the apparent authority question is a lot closer than you make it out to be. There are plenty of cases in the US where "the nature of the transaction" should have caused one to question whether the employee had authority, etc. Not just that, but in New York state, where Novus Partners is, the law is nowhere near like you make it out to be. AFAIK, under New York State law, the apparent authority doctrine will hold a principal responsible for its agent’s actions as long as the principal clothed the agent with apparent authority. Novus Partners would have had to have done something explicit to make you believe this person had authority to open source.
See Hallock v. State, 64 N.Y.2d 224, 231 (1984).
“Essential to the creation of apparent authority are words
or conduct of the principal, communicated to a third party,
that give rise to the appearance and belief
that the agent possesses authority to enter into a
transaction,”
An agent can never “by his own acts imbue himself with
apparent authority,” Id.
“[T]he existence of ‘apparent authority’ depends upon a
factual showing that the third party relied upon the
misrepresentation of the agent because of
some misleading conduct on the part of the principal — not
the agent,” Id.
“Moreover, a third party with whom the agent deals may rely
on an appearance of authority only to the extent that such
reliance is reasonable,” Id.
Source: I'm a registered patent attorney and corporate IP lawyer who has been doing open source lawyering for many many years now.