It looks pretty flat to me, and in that last photo it does look like some where marks from grinding. I couldnt say for certain, but it does look like a grinding stone or mano to me.
I don't see it. What part of the country did you find this in? Either a river or creek rock or glacial deposit. It appears you found at least one river rock possibly carried there by someone else.
Might of been used till they found the rock they wanted and then tossed this one. But I have to ask is that a divot or a chip on the 1 side? Could be a rock for making points if its a divot. I find quartzite cobbles like yours with divots on them. The orange to brown is the patina that is left on the stone after it was used. I find that patina on all of my stone grinders, hammers and anvils. Think it was used but not for long.