I will try to field your questions in order.
More than two strength training sessions per week are difficult for endurance athletes due to NCAA practice limitations.
Chrono age of athletes varies between 17 and 23. Training age varies between 1 and 4.
We stretch daily post session.
As for the time frame of adaptation for athletes being 1 to 2 months, I can only say that mine don't adapt that fast. Even with power event athletes (who tend to adapt much faster than endurance athletes) I seldom see core strength increase to the levels I feel comfortable with really loading them in the weight room inside of 12 weeks.
The exercises will vary with different densities and volumes, but the structure of the general strength training program for endurance athletes will stay basically the same all the time. I suppose that means it is really changing all the time, but we don't look at it that way.
Yes, due to the function of the spinal column and trunk flexion we tend to place more emphasis on the muscles that support and move the anterior side of the body.