Four years from now, if Italy is out of the eurozone, your vacation could be quite cheap. Even so, there are also sorts of tricks for making a month in Italy affordable without sacrificing comfort or skipping things you really want to see.
Since it is you who has been one of the forum's biggest advocates of keing trip planning and Q&A's public, so that lurkers and future travelers looking at archives can have the benefit of the info, I'll start by posting a few reactions to what you've got so far (and I will be sure to send you my e-mail address as well.
You might give some thought to putting Venezia at the top of your trip, for reasons of crowds. May is just a little bit better than June in that regard. The trade-off is ending your trip in a city that might be hotter than Venice. Some would rather deal with crowds more than heat, and I would certainly put Roma right after Venezia for that reason. The fast trains make it an attractive option. All that said, also makes sense just to go with the best air deal, which might give you a reason to go to Firenze or Bologna first -- unless of course you really want to order your trip sort of historically, chronologically, taking Roman antiquity first.
I heartily endorse your extended visit to Pompei. I will also point out that it is simple to take a day trip to Tarquinia from Rome, although were it me, I would look into the possibility of spending 1 night in Tarquinia en route to the val d'Orcia -- possibly even picking up the car in Tarquinia. I'd probably nick a night off of the val d'Orcia to do that -- but there is certainly nothing wrong with your plan as is, especially if you really crave the full 7 nights in the val d'Orcia. If you are a fan of the Etruscans, then you personally MUST go to Chiusi. It's a great highlight of Tuscany anyway.
Pistoia sounds like what you are looking for as a Tuscan idyll within striking distance of Florence. The other choice is Montecatini Alto or even Montecatini Terme if you really want to veg and enjoy spa treatments and swimming pools. I think you might be too early for summer piano concerts in Montecatini Terme, but if something like that appeals to you, check it out. (PIstoia has a big blues festival every year, but I think that's in July.)
I ultimately was disappointed in Fiesole, although I had a surprisingly good lunch and enjoyed the bijoux art museum in town (its tiny). But not only are the Roman ruins poorly reconstructed, the accompanying museum is underwhelming. I actually managed to miss the famous view, partly because many of the best viewing spots are colonized by tourist restaurants, so perhaps I'm being unfair since that's the highlight of most people's sojourn there. The town center really has taken on the feel of a coach tour depot, with huge tour groups piling in.
My favorite short-bus-ride out of Firenze is Machiavelli's house, where he wrote The Prince and from where you also catch of glimpse of Floernce, a source of sorrow for Machiavelli when he was exiled from his beloved city. I visited it with a car, but I know there is a bus that goes there. It's a just a tiny borgo nestled among vinyards. You need to e-mail in advancce to have someone show you around the house, but they are very friendly to people who have an interest in history.
With 10 nights in Bologna, I highly recommend a day trip to Brisighella. I think I'd sooner take a day from Bologna and add it to Venice and see Verona as a day trip from Venice -- or, like you're thinking, saving it for another trip (in which case you could even think of it as a hub for a visit to Trento and Vicanza and other nearby sights of interest.
Another possibility for a day trip from Bologna is Mantova, or Rimini if you are a Fellini freak (the town has other nice features) and Faenza if you have a very strong interest in ceramics in terms of art and history as well as craft. The museum there is one of the world's finest.
Padova is so close to Venice that I would probably head to Venice directly and get rid of my luggage and then see Padova on another day when you can have more time (and maybe when you need a break from Venice, if you do).