FYI, there should be a fuse in the engine compartment that you can pull to turn off awd. If you really wanted to be adventurous I am you could install an online switch after the fuse.
There is a fuse to disengage traction control. I've pulled it before and the car started acting weird, like you press on the throttle and nothing happens.
That hasn't been the case for a few years...and definitely not with the CVT.
I doubt you would see any MPG improvement since the wheels would still be spinning the diff, etc. Now, if someone made manual hubs then you might have a chance to save a bit of gas.
The CVTs use an electronically controlled clutch to transfer torque from the engine to the rear wheels. Disengage the clutch and no power goes to the rear wheels. Subaru simply programs their cars to always have that clutch engaged at varying degrees and doesn't let the user control it, hence "full-time AWD." Honda and Toyota's part-time AWD has their center clutch programmed to disengage completely for most driving and only engage (somewhat) when wheel slippage is detected. Like Subie, they don't allow user control. To make matters worse, even with wheel slippage the Toyota and Honda lighter-duty clutch will engage less than the Subaru to prevent damage to itself, which is why they can't keep up with Subies in more "extreme" off-road conditions.
The only crossover that I know that allows users to select between FWD and (part-time) AWD mode is the Nissan Juke. And that's a tiny, tiny crossover, not to mention being able to select between FWD and part-time AWD (where torque is normally 100:0 anyway) is kinda retarded.
Should be a MPG increase, but the rear wheels have more 'stuff' attached to them, compared to free-spinning rear wheels on a FWD car, so dragging them along would eat away at MPG a bit.