Mark,
A few thoughts on your most recent posts....
Where to stay on Lago di Como..... Many here like the small town of Varenna, as it has a nice ambiance and it's a good home base for exploring other parts of the mid-lake area. It's 1H:03M by train from Milan.
I suspect your drive from Milan to Chamonix is going to be closer to three hours each way. It's generally not a good idea to drive in a jet lagged state after a long international flight, a practice which has been compared to impaired driving. Rather than spending about six hours in driving time for travel to and from Chamonix (you may not have much more time than that to look around Chamonix), you might instead opt for a short stay in Stresa (no car rental required). It's a beautiful city, relatively close to MXP and more importantly on the rail line to Switzerland which will facilitate the next part of your trip to Lauterbrunnen / Mürren.
For the next segment from Lauterbrunnen to Varenna, the trip will be 5-6 hours, depending on train (if travelling from Mürren, the trip will be longer). One of the easiest trips will be a departure from Lauterbrunnen at 07:33, arriving Varenna-Esino at 13:23 (time 5H:50M, 4 changes, reservations compulsory on one segment). That would get you to Varenna at a reasonable time to check in, and then do a brief walkabout before dinner.
I'm not sure travelling to Venice for one night is going to be worth the effort. Travel from Varenna will take about five hours, which doesn't include time for hotel, check-out / in and waiting times, etc. That means you'd only have arrival afternoon and evening to look around and would have to be out of the hotel by about 11:00 the next morning. Travel back to Milan will be a minimum of 2H:25M via high speed train.
For your day trip to Florence, note that your destination station will be Firenze S. M. Novella.
On the subject of car rental, a few points to mention.....
It's important to note that for driving in Italy, each driver listed on the rental form must have the compulsory International Driver's Permit, which is used in conjunction with your home D.L. These are valid for one year, and easily obtained at any CAA/AAA office (two Passport-sized photos required, which may be provided by the issuing office).
You may also want to have a look at some of the other posts here concerning the dreaded Zona Traffico Limitato areas that are becoming increasingly prevalent in many Italian towns & cities especially Florence, which is almost saturated with automated ZTL cameras. EACH PASS through one of the automated Cameras will result in a €100+ ticket, which you won't know about until several months after you return home! This website provides more information - http://www.slowtrav.com/italy/driving/traffic_cameras_speeding.htm . I received a note recently from one traveller who received several ZTL tickets in Italy, and his wallet was "lighter" by about $700 when the dust had settled.
There's also the issue of parking tickets, high fuel costs, tolls and automated speed cameras including the devious Traffic Tutor system which measures not only instantaneous speed but also average between two points. Violate either or both parameter and expensive tickets will follow. If you did decide to drive to Chamonix, if the route you choose goes through Switzerland you'd need the Swiss highway tax vignette, as hefty fines if you're caught without it.
If you haven't used trains or other public transit in Italy before, there are also some potentially expensive "caveats" to be aware of there. If you need more information, post another note here.
Good luck with your planning!