This sometimes happens to my variegated plants if they are in stronger light than they are used to--the variegation turns pink, which is really just the backs of the leaves turning red in response to the light. I don't know if it would happen to a solid green plant or not, but I suppose it's possible.

We need a photo, if you can. SBST always lay on the underside of the leaf. Their eggs are round.

I have seen a LOT of Lambsquarters in my yard (I finally started eating them and they are way better than spinach!) and yes, I frequently see a dark red on leaves, ussually toward the stem, and the stems a deep maroon. As I recall, they are usually the smaller plants, so you are probably right about the maturity.

I can't speak for others, in my yard, the maples are an aggressive weed and their roots very vigorous. I don't notice from other trees anywhere near as much. Notably these maples can grow new trees by sending out shoots, so I have to actively keep pulling the new ones out. (Tough little ones, too) Not surprising to me that a lot of trees want their roots in the leaf pile, they think it's the forest floor. I can't say my leaf approach is special: I try to shred leaves especially maple as they mat and break down slowly otherwise (but if the leaves are too wet I don't shred). I vary my approach a bit from time to time but mostly it's pile them up and forget about it. I get piles of maybe two cubic yards - but classic pile without a bin so wider at top, thinner at bottom. Sometimes I turn them actively (but not often), sometimes just leave them alone. Sometimes less of a tall pile and just hedge-like near a fence or wherever there's some room. (Perhaps worth noting I leave some loose leaves on lawn or elsewhere, I'm not looking for a golf course lawn, those break down in place) The piles are usually almost completely broken down by the next fall or the following spring; probably would be more perfect the fall after (i.e. two years after piling up). If I want to use them before that and want it more fine, I run over them with a lawnmower. I don't mean any of this as "how you should do it", just works for me, particularly from the not work too hard at this angle.