I would consider getting the Mont Blanc pass, even if you are going to be there
for just one day. For 2+ days it's a no brainer.
- the difference in pricing is not all that much, and you may even come out ahead
when you consider that the regular ticket does not include the Monte Bianco gondola
that goes into Courmayeur.
- you make a reservation for the AdM with the pass, but you can change it with no penalty.
- the pass includes the train to the Mer de Glace as well as all the other trains and lifts
Note that the Panoramique that goes across the glacier is an additional charge no matter
what and you buy it at the top of the AdM (maybe at the bottom also, but def at the top).
The one disadvantage I can see is if the weather is completely socked in for your entire
stay, then you bought a pass and it's wasted. But, you could also wait until 2-3 days out
and if you buy something, buy the pass instead of just an AdM ticket.
In the plaza where the AdM is, there is a small kiosk/booth where tickets/reservations can
be purchased and changed. They are very helpful.
In my experience, the crowds are larger in the morning and often you can just walk on in
the afternoon. But your proposed itinerary is a much longer one (there is often a wait for
the Panoramique), so you would want to start mid-late morning to be able to enjoy the trip.
Note that you don't need reservations to come back, but you may have to queue.
