no_style, welcome to EpicSki!
Well, I'll take a swipe at your questions, but some here will argue with my answers:
1) If you can buy a binding without a ski, it is designed to be mounted on any ski that doesn't require a specific binding (and that's narrow enough for the brake to still work). All manufacturers of bindings make bindings that will mount on "flat" skis.
2) Lifespan has to do with two things, primarily: breaking of parts and fatigue of the springs. Fatigue takes 5-10 years. Breaking of parts has more to do with use than anything else. Fatigue is the reason you want to have your bindings tested each year.
3) There are no "best" or "worst" bindings on the market. I have skied most of the major brands over the past two seasons, and they are all good. You may prefer some features more than others, and that may focus you in a particular direction. For my part, I find skis that I like, then buy bindings that will go with them. For example, I prefer some of the features of the RailFlex 2 system (like ease of mounting and adjustability), so I bought my Fischer RX8s with the RailFlex integrated binding. Last year, I bought them flat with Marker 1400EPS bindings. I also like the way that the RailFlex feels when I ski them a bit better.
4) Those qualities that you most care about! For example, much has been made here about the weight of the Atomic Neox bindings. I've been skiing them this season on my Metron:b5s. I really like them. They are solid, both in terms of general mechanical characteristics and in terms of materials. I like that. I couldn't care less that they weight more than (for example) a Rossi Pivot. For others, they'll avoid them for that reasons. I also like that both of the bindings that I have this year (the Neox and the Fischer/Tyrolia) have adjustability built-in. I have used it on the Fischers, but left the Neoxes in their "Forward" setting.
My view of bindings? They are the last thing I think about when buying skis, and I rarely think about them after they've been purchased. I'll just ski on them. I tend to be pretty quiet on my skis, so I don't have issues with pre-release or the like, and never have. I used to be able to feel my Looks re-center my boots on my skis back when I skied them. That was kinda cool. The Markers, Atomics, and Tyrolias seem to recenter faster than the old Looks did...