You'll need both the ground rods and the feeder cable ground wire
The ground wire in a feeder cable to an outbuilding performs a very different function to the ground rods at that outbuilding, and you need both functions to work correctly to have a safe electrical system. In particular, the ground wire in the feeder returns man-made electricity back to its source (the utility), while the ground rods are for returning natural electricity (translation: lightning surges) to its source (terra firma). In addition, you will need to pull the bonding screw in the panel, as neutral and ground are only connected at the main panel.
As to the 100A main breaker in the subpanel, you are correct that it is basically a shutoff switch.
As to that overhead cable run
There are two ways you can do this: either using UF for the entire run, or using wires in conduit for the masts with a service drop quadruplex cable for the overhead portion. Either way, the overhead run needs to be at least 12' above ground, and the messenger wire (or bare wire in a service drop cable) needs to be attached to a secure anchor point on the structure using fittings identified for use with overhead service conductors (such as a fork bolt).
The one-cable approach
This approach uses a 6/3 W/G UF (Underground Feeder, "outdoor Romex", not to be confused with NM) cable for the entire run. You'll be shoving it up conduit (RMC or Schedule 80 PVC) masts on the outside of the buildings with a drip loop at the weatherhead, where it exits and transitions into being supported by a galvanized, copper-clad, or stainless steel messenger wire between the two buildings, and is attached to the messenger wire either using cable saddles that clamp onto the messenger, or by using a field-fitted lashing wire wound around the combination of messenger wire and UF cable. I would recommend the cable saddles, as it will be easier to get a clean install with them, and also less problematic for the rather...flat profile of a typical UF cable. You will also need to ground the messenger at the house end, using a Burndy KSU or equivalent split bolt tied to either an 8AWG bare copper ground wire (in a PVC conduit) or an 8AWG bare jumper going off to a Burndy GC15A or equvalent universal pipe grounding clamp.
Going aluminum
If you want to use aluminum for this, the best route would be to use an overhead quadruplex cable for the feeder with individual wires in RMC or Schedule 80 PVC for the feeder masts and no messenger wire as the quadruplex cable has a bare wire in it already that will act as the messenger. If you are using RMC, then a 1" conduit suffices for 3 4AWG Al XHHW-2 conductors (or 3 6AWG Cu conductors, either THWN or XHHW-2). If you are using PVC in Schedule 80, then I would go with the same options for insulated conductors as before, but with a bare 8AWG copper ground wire in the conduit (to keep conduit fill manageable, especially with aluminum XHHW-2s). As to sizing the quadruplex itself (which provides its own messenger), a 6-6-6-6 (codename: Chola) Al service drop quadruplex is amply sized for a 60A overhead feeder (think about it: a wire in open air is going to be cooled by every passing breeze, something a wire in a cable in a wall or encased in conduit doesn't get). You will also need splicing supplies to join the cable to the mast wiring at each end, namely Cu7Al insulated two-port splice kits such as the Burndy AGS-2, Cu7Al universal split bolts such as the Burndy KSU series for grounding the messenger, and universal pipe clamps such as the Burndy GC15A for grounding the messenger to the mast if you're using a RMC mast, as well as 8AWG bare copper grounding jumpers to connect the split bolts to the pipe clamps in that case. Don't forget to leave a drip loop in the overhead cable connections, just as if you were wiring a service connection.
TORQUE ALL CONNECTIONS TO SPEC
In addition to being a new Code addition in 2017 (NEC 110.14(D)), it is simply good practice to get a calibrated inch-pound torque wrench and use it on all panelboard and breaker lugs and mechanical splices, torquing them to the manufacturer's specifications. That way, your connections won't go off and leave you looking like Greg Biffle's loose lug nuts.
