I have a completely different take on this.
It is not all that unusual to communicate "outside the system" for certain purposes. It is true that you lose some protection, as Air BnB cannot read the messages, and there is no record of it should you need to resolve a dispute. But if you are requesting certain specific things to be provided, like extra sheets or so on, as well as arranging a time and place for key pick up, the owner would want to include the property manager in the message exchange so nothing gets lost. For the apartment I rented in Switzerland last summer, the owner gave me the phone number of the on-site property manager, and I was able to contact her directly about various things. Air BnB did not need to be part of these communications. It is not weird or concerning to do this.
With a Super Host with 90+ great reviews, I would not feel at all uneasy communicating directly.
As for the offer of a discount if you pay her directly, that is not necessarily a scam, or anything other than a sincere offer on her part to save you both some money. I personally would decline such an offer, as did the OP, but I am quite sure some people take her up on it, and it probably works out fine, or she would not have offered it.
If you are uncomfortable proceeding because if this, go ahead and cancel your booking. But please do not "tell on her" to Air BnB. She has done nothing fraudulent and does not deserve to be treated like that. Assuming it is a legal vacation rental under Parisian regulations ( and of course the OP would have vetted that), she has considerable time and effort invested in her business. Losing her Superhost status would be a serious blow. Do you really want to do that, when she has caused you no actual harm at all?