You've chosen plants that are about as tough and durable as plants can get. That being said, individuals (referring to plants) can have differing results. Also true of people.

Color of cambium?

My bet is too much water and/or planted too deep

Well.....it's dead. I scratched the bark on the trunk, and it's brownish/white. I dug it up and inspected all the soil in the planting hole. I ran my hands through all of it. The soil wasn't at all wet, it was damp. It was planted so you could see the root flare.

I'm disappointed about this one; it's almost impossible to get a multitrunked amur maple, unless you want to pay Sooner's ridiculous $100, which I won't.

I'm going to look at other types of maples to plant. I know I could put a sugar maple in there and it would grow effortlessly, but I don't want that big of a tree. Thank you for your replies.

Just saw 4 ft. with short single trunk and multiple leaders Amur @ Lowes for $50 last weekend. They looked ridiculously healthy.

I just ordered another one, at Cold Stream Farm, for $5 ($17 shipping). It's a single trunk tree, but I can prune it later on if I want, to make it a multi trunk, since it's only going to be 1-2ft tall, and bare-root. The one that I planted 2 weeks ago was so root bound that it wasn't funny. At least this one won't be...

It's worth a try, and if this new one doesn't work out, no big deal, I'll try again with another type of tree next year. Thank you, Bill and Smivies, for your responses.

I’m in Nebraska and the tree nursery guys aren’t even telling me it’s ‘ok but not recommended’ to plant the firefall maple I am going to have planted on the western side of the property to provide future shade, until the end of August. They simply don’t want to see the tree transplanted only to get burnt and die over the course of the summer heat - and I’m getting a tree that is several years old already, the nursery will be doing the transplant and irrigation setup.