yea, it can def get below 40F at night, it all depends on where and you're elevation. I used to take my mummy liner in the summer which is/had generally been enough for me, but it weighs as much as my quilt and I got screwed one night where it got too cold to sleep. I may or may not have stolen my dog's body heat that night (what happens in the White Mountains stays in the White Mountains?).
there are plenty of well maintained stealth sites all over the Whites. I've talked to shelter workers and park rangers before and they've all told me the same thing: if it's there already and it's well taken care of by hikers, we'd prefer their continued use over the creation of new stealth sites. you can't stop stealth camping, so it's better to have it controlled to this extent than to be the enemy, which only makes the situation worse.
that said, hammock hangin' ftw!