Hi Sharon,
I can only comment on the two area's we have been to. I would actually add a night to Meran/Merano, but probably stay a little further west in the Vinschgau/Val Venosta. My opinion, I think you should omit one place, as the three 3 nighters are only 2 days in each place. If you're looking to relax and do some hiking and/or walking, these activities are weather dependent. An extra day increases your chances of a good weather day.
Also, it would most likely be a lot cheaper to pick up and return the car in Germany. I'd consider taking the train from Munich to Garmisch, get the car and then return it in Garmisch and train to Munich. Garmisch is an easy 2:30 drive to Ortisei.
We stayed in the Val Gardena once and then visited from our base in the adjacent valley (Alta Badia) twice. We also stayed in the Vinschgau/Val Venosta, which is a valley that runs basically from Meran/Merano to the Swiss border near Mustair, about an hour away.
These two area's are quite different. You'll find more vineyards in the Meran/Merano/Vinschgau area than in the "Dolomites", which are basically between Ortisei and Cortina d'Ampezzo. The alps in the Dolomites are associated with the towering limestone peaks and the Vinschgau is more like the alpine scenery you'd see in Austria and Switzerland. You will not see anything like the Dolomites in the alps. Dramatic is an understatement. The Vinschgau, along with alpine scenery, has plenty of medieval castles (Tirolo, Juval, Kastelbell, Churburg, etc., etc.) and then Glurns/Glorenza, a perfectly preserved medieval walled old town, about 45 min. from Meran/Merano. Near the Italian/Swiss border is the Convent St. John, in Mustair, Switzerland. Certainly worth the drive to visit.
We could easily spend 5 nights in either the Dolomites or the Vinschgau/Val Venosta. We spent only two in the Vinschgau and 11 in the Dolomites. 4 nights based in the Val Gardena and 7 based in the Alta Badia.
Hope this helps.
Paul