I really don't think this is Pyrus calleryana. We see a lot of escaped callery pear around here, and in my experience the escaped trees always look pretty much like the variety 'Bradford.' This shrub looks quite different. You can't tell from the photo, but the shrub's leaves are somewhat hairy, whereas Pyrus calleryana leaves are hairless. Pyrus calleryana leaves are also glossy, somewhat leathery, and toothed. The mystery shrub leaves, in contrast, aren't glossy or leathery, and they are completely untoothed. I would believe that an escaped Pyrus calleryana could revert to being thorny, but I've very doubtful it would change all those leaf characteristics.

That is Persicaria 'Crimson Beauty', a gorgeous, non invasive relative of the wildly invasive Japanese knotweed. I have had one for 6 or 7 years, and not one seedling or root movement, and this in sandy, light soil. However, mine, in zone 6 begins bloom in early September and the whole show is over by the end of the month. Fully zapped by frost by now.

There certainly are lots of species popping up that were never there before - including what looks like a grevillea species that's just started to appear. There's a guy from the council who's been helping people with advice on revegetation & I've heard he's gotten really excited by some of the plants that are appearing. He's got a blog with pictures of some of what he's found in his work - I'll attach the link for anyone who's interested. http://notechis.blogspot.com/

@macranthos, yes there are quite a few river birch in the neighborhood, some within a block. Thanks for pointing that out, I didn't know what kinds of trees those were before your comment. And it probably explains this other volunteer I have (picture below), which seems likely to be Betula nigra. Interestingly it has the same red stem as the smaller seedlings I originally asked about, which look a great deal like the photo you posted. And yes, I also have them all over the place in my back yard and deck container pots.