A: Yes, drive to MSM. For contrast with the relatively modern Ancient Regime chateux of the Loire, stop at the ruin in Fougeres and have a Bretaigne lunch of gallettes and cider nearby.
A: 1 - 3 nights in Bayeux. If you are not interested in WWII sites, Bayeux may or may not be the best base for you to explore the region. Another candidate for an overnight: Honfleur.
A: Staying in Giverny is possible, if you go. Personally I would try to organize things so as to end the day in Paris.
A: Where extra day is a very personal decision, up to you.
OK, now for some unsolicited suggestions.
Consider proceeding directly to Amboise upon arrival, saving Paris for the end. Why? To consolidate your time there and eliminate a travel day.
If you do this, and the Ancient Regime chateaux of the Loire leave you breathless and wanting more, then by all means make time for Versailles later in your trip. If they leave you cold, skip Versailles.
Conversely, if you can't make yourself go to Amboise right off the plane, visit Versailles when in Paris. If you just love it then go heavy on the Loire chateaux. If not so much, consider cutting your Loire time short.
Three variations on a Giverny theme: Drive there from your penultimate Norman destination, when done return the car in Vernon if possible, take train to Paris.
Or, if ambitious, get up early, drive to Rouen (charming Norman city), spend several hours walking the old town, drive to Giverny, etc. as above.
Or, drive the Rouen the night before, return car, sleep there, walk the old town, take train to Giverny.
A final note: The gardens at Giverny are lovely. And very crowded. There's not much more to see there, though the countryside is beautiful and you could take a little hike. You probably know that there are no paintings there.
Have a great trip!