With a Mariner you get the best of both worlds. They track straight when level yet turn readily with a slight lean. A Swedeform shape is just one of the many aspects we use to enhance responsiveness in a Mariner kayak's handling. Your sliding seat fits me perfectly. Unfortunately no, we can make very limited numbers of them and therefore we seem chronically in danger of running out and holding up the delivery of a customer's new kayak for want of a seat.
Secondary stability is a lot harder to define.
One designer used to claim there is no such thing as secondary stability and any kayak that is more stable initially will be more stable at all angles of lean. Personally, I define secondary stability subjectively as how secure you feel when you have leaned the kayak well to one side. This defines secondary stability as being somewhat in an inverse relationship to initial stability. Any abrupt changes would be like putting a speed bump or flat spot on the rocking chair rockers. The above definition is from my own observations and guesses regarding secondary stability in kayaks.