There are so few who have done it, and the number of posters here is a tiny fraction of the regional airline pilot population, so you probably won't get much, if any, help for a little while until more go through it. But the best guess I can give you is, if it is an in house CTP, your seniority number will be given for the date you are hired (when you start working for the company, not when they agree to hire you). If they pay for you to go to one of those flight schools to do it prior to your class date, your seniority number is probably only going to start when you get on property (i.e. start your company indoc class).

I'm guessing they incorporate it into indoc or some other time at the beginning of class, because they aren't gonna pay for you to go thru sims only to then hook the written and then have to fire or retrain you.

No guesses for the rest, but all my mil buddies who did sheppard air prep got mid-high 90s and were in and out in 30-45 mins. Thats after studying (memorizing) for 3 straight days or so. If the gouge that shep air has is still good since these new shenanigans came into being, I'd do all that memorizing before you start and then refresh before you take the written. While the material that is covered in the CTP is important to know as a pilot, the written is the written, and there are tried and true ways of taking it and acing it...unless they changed it significantly with the new one, and the old tried and true methods are no longer valid. The support at shep air could probably answer that for you.