Mother Nature interfered with my plan, as she is wont to do. We had some major flooding in my area and some poor folks literally lost their homes. I was lucky in that regard; only my basement flooded. I didn't feel lucky at the time, of course, given it knocked out the furnace (and temps were below 50) and the water heater so we had little heat and no hot water for a week. Got that taken care of as much as is possible at this time and am now getting back to my summer plan.
I did the first room. HOLY COW, what a mess. Covered nearly from head to toe with a Tyvek suit, goggles, respirator, gloves, shoe covers ... and still managed to be coated in tiny little droplets of that stuff. Covered the floors with plastic and still managed to get it all over the floor. I was, as you told me I would be, astounded by how much it expands! Unbelievable. I did pretty well at maneuvering fast enough to prevent the hose from clogging due to inactivity and only had to change the nozzles twice. Even with vaseline on the nozzles, though, it still gunks up pretty quickly, but wipes off easily if you wipe it fast. So for the next room(s), I'll put on a belt over the suit and tuck in some junk rags to wipe the nozzle after each cavity. I'll also fashion some kind of headband to protect the exposed inch of forehead between the Tyvek suit hood and the goggles, and wear a long sleeve shirt tucked into the gloves, under the suit. It's hot in that suit even with a tank top on, but the difficulty of getting the little droplets off is worse.
Due to your excellent feedback and suggestions, I decided to just do the 1" of foam and then layer with batts for additional R value. I already had 4 rolls of insulation batts that I bought before I knew you could do foam yourself, and it seemed more cost-efficient to go this way. I checked their website and they do list this as a possibility and even the best choice in some situations, though of course they recommend just buying more foam and doing full-foam the majority of the time. I can't help but believe that's at least in part because it would make them more money. So, I feel OK with foam + batts and know that even if going full-foam would give me a little more R value than foam + batts, this will still be dramatically better than what I had before and my pocketbook will thank me.
Now, to carry all the sheet rock up there and get it up. Then, tape + mud. Then, start tearing down plaster and lathe in other rooms and start the process all over again. Sighhhh. If anyone finds themselves headed to Vermont and inexplicably wants to spend their time doing hot, sweaty, tedious work, just let me know. :) Thank you all so much for your help with this. I often feel completely alone and my few friends/acquaintances think I'm crazy to be trying to do these kinds of things. My poor mother begins hyperventilating at even the mention of my using a tool or anything that has chemicals in it. It's nice to have some people who don't act like it's insane to do these things yourself!