I'm from just north of Maine, so I know the climate well (and I've been to Katahdin via Chimney Pond and the Knife Edge).
OK, bug defense is going to be a major issue for you until August...that definitely has an impact.
So, for daytime, a lightweight nylon shirt (hiking shirt) will likely be "biteproof". I don't know about you, but I hate applying DEET, so I try to do as much as I can with barrier methods (long pants, long sleeve shirt, bug net, etc.). What can I say - June is hell there!
Undies: I think bike shorts are going to be hot. Many of the gals here are fond of Smartwool (or other brand) merino panties. Very lightweight, non-odor-forming; dries quickly.
Camp Shoes: Usually in ME, NH, VT, and NY there aren't many creeks to cross because there are footbridges. Maybe they've been washed away and not replaced now (like Marcy Dam in the Adirondacks)? I think you could cross creeks in your (non-Goretex) trail runners, take them off, wring out your socks (or put on a dry pair), and continue on without blisters -- but only time will tell. There has been I like camp shoes, but I'm not UL (only "L"); the majority here tend to argue against them.
Water: I've been using a Steripen-like device since before Steripen existed (but that first one - I forget the CO company that made it - did break in the field). So far, I've had no troubles with the Steripen brand (I've upgraded twice to newer models). I always bring clor. diox. drops as a backup. And if you're really squeamish about water -- then "double treat" until you become more comfortable (filter, then drops or boil the same water). You are in beaver country, so you want to be careful. And with a bit more experience, you'll learn to trust the process more. Give yourself time.
Not sure if you've seen this cheap down bag (http://www.walmart.com/ip/Ozark-Trail-32-Degree-Down-Mummy-Sleeping-Bag/22008955) - since you mentioned Walmart, I thought I'd mention it. It's been discussed on BPL, Hammock Forums, etc., and folks generally like it. It would be quite compressible, and should be fine for summer nights in ME until mid-Sept in a normal year. But I would still try to haunt GearSwap to get a bargain on a 3-season better-quality, lightly-used down bag! And I 100% agree with Roger's advice about NOT believing the temperature ratings on sleeping bags! I am a cold sleeper, so I offset most manufacturers' ratings by 15F to 20F degrees (so, a bag rated at 15F by Marmot is a 30F bag for me) -- then I know I'll be comfortable.