Yes, this will last for multiple years in a garden or flower bed.
In theory no, but I think you would have to make sure that the fabric stays fairly clean. Over time dust would settle on it and that along with weed seeds would probably lead to some germination and weed roots will grow through the fabric once they get started. The mulch/rock acts as its own supplemental barrier so just having the weed barrier would give you less protection than having both the barrier and something on top of it. Just my two cents as I haven't tried it myself or seen it done.
yes, water does go through. It depends on what material is covering the fabric, and the amount of rainwater, but in general water will penetrate the fabric.
I'm unable to mic this fabric and the thickness is not defined on the label. I can tell you, it's about .50 mil, its definitely thicker than heavy weight garbage bags. Hope this helps.