On my 17 signature i feel too less breaking then my other cars but not terible. When my brakes go i will install ebs brakes and vented rotors also get a wider break line.
On my Signature I only use for traveling and on freeway w out camper I get about 25 MPG in flat areas, also I use non ethanol not sure if it makes any difference.
As far as the humming sound it might be something simple as rotating tires.
Best way is pull the dipstick and put some oil on a clear plastic and see color, but 7,500miles is fine to be safe, thousand miles over or under wont hurt it.
I have not seen any neither, what I would do is take them of and see if its superficial rust you can spray some pb plaster and or use very light sand paper to see if it will come off.
No noise i got it to about 110 and felt very solid , excellent steering feed back, am use to audis and infinitis, test drove about every suv sub 60k and found cx9 signature to be most fun overall.