There is absolutely nothing to see or to do in Liechtenstein and at Limmernsee.
If yoou like a Disneyland castle and long queues, go to Neuschwanstein. But be aware that this castle was built in the 19th century (in a time where castles were of no military utility) and could never have been defended. If you wnat to see a real European castle (fortress), chose a medieval one; there are plenty of them along your itinerary.
There are comfortable direct buses from Venice to Cortina d'Ampezzo. A car may make sense for the trip from there via Falzarego Pass - Alta Badia to Ortisei, however.
Public transport in South Tyrol and Switzerland (from Toblach/Dobbiaco up to Lake Geneva) is excellent. Trains or buses run every 30 to 60 minutes. No seat reservation needed.

Must you fly back from MXP?
If not, you could drive from Ortisei via Bolzano (Oetzi museum) - Tyrol castle - Stelvio (highest Pass road of Europe) - Mustair monastery (medieval Unesco site) - Upper Engadine (many cableways to glaciers, like Diavolezza) - Lake Como - Lake Lugano - Bellinzona castles (Unesco site) - Locarno to Stresa, Pallanza or Domodossola, drop off your car and go on by train to Brig castle - Zermatt - Interlaken/Grindelwald and fly byack from Zurich.
Otherwise drive from Bellinzona to Giornico churches - Nufenen Pass - Grimsel Pass - Interlaken/Grindelwald - Kandersteg - Loetschberg railway car shuttle - Loetschental Valley - Zermatt (Taesch parking) - Brig castle - Simplon Pass - MXP.