Depending on company and your corporate rates typical midsize rates in Northern It run around $50 per day, all in. The cheapest options tend to be from Sixt or special corporate rates from Hertz/Avis. Be very careful about extra charges in Italy, there're often more than the base rate. Be careful too about distance charges. the allowances tend to be small and extra km are costly.

For Venice definitely return the car and rent another. The incremental cost will certainly be less than the Euro60 for a three day parking plus the car rental itself.

You can buy the stamp on entry to Switzerland or just stay off the toll roads and you'll not need the sticker.

You will need a gps if you plan to drive in the cities. It will save you money in comparison with the time you'll otherwise spend being lost. I never go anywhere without mine and I sam in in Northern Italy six or more times per year.