The more I look a the key, I think you are right. The only species as a nymph close to it is the Oecanthus niveus. this guys wings seem a lot narrower too.

Would a tree cricket be in the grass?

...considering that the only 'tree' crickets I've ever been able to catch were all within a foot of the ground in some scraggy old bayberry. These were perfectly well-flighted adults, too, not nymphs that might have fallen down off nearby trees or shrubs. I've also found adult female 'bush' katydids walking around on the ground between weedy grass tufts and Carolina grasshoppers basking on sunward-facing spruce boughs about four or five feet up down by the beach. The only conclusion I can draw is that whereas each species may have its preferred habitat for feeding and breeding, when it comes to just sitting around, I guess that, like a 500 pound gorilla, they just sit whereever they darn well please.

through the tallgrass prairie of Somme Nature Preserve, and interestingly asked the same question!!