Register an art historian business in France?

My wife needs to register with URSSAF so she can get a business SIRET number here in France. She is American (as I am) and is an art historian. We are residents in France, retired, have French Social Security numbers and Carte Vitales. She is an author and gives lectures on art history subjects for which she gets an honorarium. She receives small amounts of Droits d’Auteur for several books already published.

She has been asked by the Réunion des Musées Nationaux to curate an exhibition and needs a SIRET number. We have been advised by the local tax office (who admitted he was not an expert on this registration) that she should become a micro-entrepreneur using APE 99990 – Activité non-commerciale diverses (whatever that means) or 90003 Droite d’auteur non rattaché à l’activité... (can’t read his handwriting). I think these are both in the area of profession libérale.

Questions:

1. Should she register as a micro-entrepreneur through URSSAF?
2. Are APE 9990 and 90003 under profession libérale?
3. Are either of these the correct way to categorize her profession?
4. We were told by this tax official that she should be considered primarily a conférencière, which will cover all three of her major professional activities for which she receives some revenue: Auteur, commissaire d‘exposition, et conférencière. Your thoughts?
5. Since we already have Assurance Maladie (and I get a small French pension) we do not need either of these – so is there some way to avoid paying into these systems?
6. What is the proper web address to register online?