This boat falls under the category of "sit-on-top" kayaks. One of the pluses for this style of kayak is that they are very stable, and in the event of a capsize, they are very easy to climb back on top of. The structure of the boat is hollow with few openings into the inner chamber. And to answer your second questions simply, it would be very difficult to sink this boat. However if the front hatch (gray and black rubber piece) is not secured properly, the boat could sink more easily.

This boat has adjustable foot braces that should be able to adjust far enough for you. However the only true way to know is to try sitting in the boat, or find measurements and match them to your inseam.

No, this is not a pedal kayak.

Yes the pedals do slide and are easily adjustable.

A lot of times, back pain can result from incorrect posture as well as the kayaks outfitting. when you're paddling it can be very helpful to keep your feet on the braces and pushing to keep your back against the seat back in order to have a "dinner-table" posture. To answer your question, this seat does have decent support, however posture has a lot to do with back pain as well.

This kayak has quite a bit of support, and it good for all levels of paddlers. The back is almost like a chair, with very good padding to keep you in the saddle longer.