What distances can the 4 conductor 18 Awg be used for within signal degradation?

It mainly depends of the strength of the signal being sent through the speaker wire. But in general it should run 300 - 400 ft.

Has anyone used this under padded carpet? Thinking of running along the cement slab in my basement under padding and carpet.

I ran mine under carpet worked good.

Can you run usb 5v dc power over this wire?

USB maxes out at 16ft, without a repeater. 1.5Amp or higher USB shouldnt be run more than 10ft. As per USB specs. Splicing your own USB is just asking for problems from the controller.

What is new or different about version 2.0?

The backing is slightly textured on version 2.0 which is much better to paint over and less prone to permanent wrinkling than the original. The adhesive is also a lot stronger while still being pretty easy to terminate since one side of the copper is clean.