Day 1: paris to caen,hire car + D-day tour , eveninng at MSM and stay ther
Day 2: MSM + stay in malo or near Rennes
Day 3: Rennes to vannes , stay at HOUAT drop the car.

Day 4: Houat- vannes to paris late evening train.

is this do-able ?

I can't speak to Days 2,3 and 4, but Day 1 seems like too much. It sounds like you are doing a self D-Day tour? The D-Day sites are somewhat spread out...I don't think you are going to have enough time to do them justice and travel from Paris to Caen by train, go to a car rental place, then get to Mont St. Michel in the evening after touring...it makes me tired just thinking about it. Most D-Day tours take off from Bayeux if you are actually booking a tour, which I highly recommend. I would stay in Bayeux (which also has a wonderful cathedral and The Bayeux Tapestry) the first night, after the D-Day tour, then leave the next day for Mont St. Michel.

Not really. Do you have a map? Check the distances. You're biting off way more than you can chew.

He's posted this before I think.

When you have a car, everything is possible if you drive efficiently and limit the time of your visits.

Check out your itinerary on www.viamichelin.com which will give you driving times.

C'mon Kerouac. Yes, "anything" is possible. Is it realistic? NO.