Hi there,
Well, I think that's the most well planned itinerary I have ever seen, heh heh heh! If I were to give my advice (and based on what you said yourself - that you want to go with the flow) I'd suggest just ripping up the day by day plan, and deciding what places you definitely don't want to miss, and perhaps just have dates (if you have to have any that are) set in stone on a country by country basis. You might find there are places you want to spend less time in along the way and those you want more time in.
One thing to note is that I'm pretty sure you have to enter and exit Myanmar / Burma in the same way, i.e. if you fly in, you have to leave from same airport or if you go by land, you have to enter and exit by the same border point.
You had a question mark next to Sihanoukaville in Cambodia - that's a great little place

I was surprised not to see Chaing Mai on your north Thailand itinerary? There's also some treks etc you can do round the traditional hilltribe areas like Pai if you like that sort of thing.
I'm not sure about American passports and visas by country, but I got my Thailand and Laos ones at the border crossings I chose, and both are good for 30 days. You needed the Cambodia one in advance when I was there, unless you flew into the country, which I did as I was running out of time.
Hope that's of some help...