Ok, I coated a piece of scrap foam last night with DeckOver and just subjected it to some stress testing. The coating is slightly flexible and is more impact resistant than uncoated foam, but I don't think it would qualify as a hard coat. An impact with a hammer that broke the surface of the uncoated foam just left a slight dimple on the coated side.
I just put thin coat on them but the surface felt harder than the foam and I think it had a little give to it. I still have some left so I'll put some on a scrap piece of foam and see just how durable it really is.
How about a thin-set mortar like this. I haven't tried it as a foam coating but I don't see why it wouldn't work. You could even mix it with some exterior house paint to make a waterproof monster mud-like coating.
For me the Halloween season starts November 1st. That's when I start taking down that years decorations and planning what I'm going to do next year. I work on new props and decorations all year long as time, money and inspiration permits. It usually kicks into high gear around August 1st.
I like the first arch as well and you shouldn't pass up on getting it just because of a slight mismatch in size. If the arch is too narrow to fit over the sidewalk you could always anchor it directly to the concrete. A masonry drill bit and some anchor bolts would secure it in place. You could also just add some extensions to the base that would extend over the edge of the sidewalk so it could be staked down.
Edit: I guess I shouldn't have answered the phone in the middle of composing my...