Critique 24 Day Itinerary

I would drop Chamonix, as your heading out of your way just for these two days.
Divide your alpine time between the Italian Dolomites and the German/Austrian Tirol.

Arrive Milan
Drive to Stresa on Lake Maggiore 2 nights
Drive to Cortina d'Ampezzo 3 nights
(Short Stop in Innsbruck)
Drive to Garmisch Partenkirchen - Tour Mad Ludwig Castle 3 nights
Drive to Venice - 4 nights
Drive to San Marino - 2 nights
Drive to Tuscany - 6 nights - relax - tour small towns
Drive to Rome - 4 nights

If I was to forgo Chamonix, why wouldn't I drive to Garmisch-Partenkirchen via Liechtenstein then down through Innsbruck to Cortina d'Ampezzo then on to Venice... this way I wouldn't be backtracking and would see Liechtenstein?

Yes, that would definitely work as well. Lugano or Como may be better lake options accounting for the ease of transport links. Remember that Liechtenstein has little to hold your attention for very long.

I don't remember any place worth a stop for lunch in Liechtenstein. Probably you would better have lunch at Heidiland (a rest station on the motorway) or in Feldkirch, Austria (much nicer than Liechtenstein). Remember that the Arlberg tunnel is closed for maintenance till mid November, so do not include it in your itinerary if you are doing it now. - In general there is too much driving and those 2 night stays are really 1 day stays.
San Marino is not particularly interesting - Urbino or Ravenna would be better.

Thanks, but which 2 nights stays are only worth 1 night?
Jean