EmilyAnn, first-- the most important thing-- happy birthday to your DS!
Next, yes, it sounds to me as though sticking with Kraft/Nabisco and Keebler brands for most of the things you have listed would be safe for most people with peanut and tree nut allergy-- assuming, as you already know, that the labels read safe. The prepackaged snacks you've got listed are things that I've heard others refer to as "go-to" snacks-- I'd ask about them up in manufacturers, but presumably you use them regularly, yes? Another idea for pudding is Kozy Shack.
I'd ask the two nut-allergic families what their comfort is with the food, though-- just make a note that your DS is also nut allergic, and ask if they plan to have their kids eat the party food, and if so, should brands X, Y, and Z be okay with them.
As for when this gets easier, it's sort of an asymptotic curve-- I found that the steepest improvements were in the first six months, and about that much over the next two years, and about that much over the next six years...
I hardly notice it now, honestly. DD was recently AMAZED when we ran into some friends at the grocery store-- "Mom-- it's WEIRD, they look at the prices but not the labels!! We spend a ton more time looking at the labels on everything! Is that how everyone else shops??" She's never known another way.