First off, you are all so kind, thank you for your answers and further questions. Here are my answers/thoughts:
Laura: I believe I will commit to Venice-Florence ahead of time to save the money. Regarding the Swiss Travel Pass - good idea, that is another topic I need to post about. I need to get advice on whether that makes sense for us, look for my post!
Donald: Thanks for the comment about long lines at ticket counters; that is one of my worries. But if I get a ticket ahead of time, don't I often have to reserve a seat anyway, which requires standing in a line? When are reservations made? I am uninformed on this point.
Lee: Sorry about the "trip two two" typo, I was mortified when I saw that later. In any case, the information about the Einfach-Raus-Ticket is wonderful. Is that good on IC trains as well as regional? Because I only see IC trains from Attnang-Pucheim to Salzburg. We were actually going to continue on from Salzburg to Munich late that afternoon, so that ticket would be a real moneysaver if I could use that same one. Or is it invalid for travel from Austria to Germany?
Ken: I was originally planning a night train from Munich to Venice, but we changed our minds because airfare is so cheap. So I have already purchased tickets for that leg. I will definitely buy the ticket ahead of time for Venezia to Firenze. That trip will be Monday, July 4. When will tickets go on sale? Regarding the bus from Florence to Siena, is that available ahead time, and on what site? Regarding the Camogli-S. Fruttoso to Lauterbrunnen trip: I considered buying the ticket from Milan to Spiez section ahead of time because it's an EC train, but worry the regional trains might be late. That trip is on a Sunday, July 10 so don't know if that makes a difference. Regarding Spiez-Lauterbrunnen - I am considering various Swiss passes, and will post on another topic about that. THANK YOU for your reminders to validate tickets, something I hadn't considered. You provided really great information, thanks.
Chiani: You are correct in that we don't have a lot of flexibility. I was planning to buy my next ticket when I arrive at the station, and in that case you are correct that I won't know any better than I do now. Quite honestly, I would choose all my trains now, except that I am worried that my late-sleeping teenage daughter would make us miss them!
Traylaparks and Valerie: I think I will go ahead and purchase most of the tickets, given the discussion here. I can't quite figure out when they are available. We are traveling June 29-July 14.
Thanks everyone, it is much appreciated. Judy