Well, the answers to those questions are major plot spoilers, if you still want to know then someone will tell you.

Tidus isn't necessarily the reincarnation of Shuyin. They just look the same. Shuyin and Lenne are situational characters that juxtapose to Tidus and Yuna. Both pairs go through very very similar situations, and their lives are suppose to parallel for dramatic effect, and to show a similar situation with similar obstacles so that the obstacles that couldn't be overcome in the past situation can be overcome in the present. And, they play interesting roles in the game anyway. It's similar to how Laguana and Julia juxtapose to Squall and Rinoa, but in this story it's far more apparant.