With regards to new hire training what checking events are jeopardy events vs events that are re-trainable?

Are the exams open or closed book?

The practical portion of the checkride is a jeopardy event. The most important thing I can say about the checkride is to treat it like a regular flight meaning use your partner. It's a two pilot operation and that's what they want to see. I'm not sure of your background but I know for me personally I caught on to that throughout training. I came from instructing so checkrides usually meant me doing everything traditionally. As far as exams, indoc is open book and everything else is closed. You'll be fine as long as you study and the instructors will point out the areas that you really need to know.

If you have a bad day and bungle a couple things on the checkride, is Endeavor's attitude "sucks to be you! Bye bye!" Or will they give you another opportunity to nail the thing you messed up on??

I never needed it but I saw countless examples of the training department going above and beyond to help people out. They hired you and they are not looking to toss you out if you bust a check ride

Kinda depends. If you have a good attitude with a good training record I'm sure they would just consider it a bad day. The instructors are very good at training and will not send you for a check ride if they don't think you're ready. Especially since it's a pria event. I have always felt that they over prepare you in that the check ride seems easier

Indoc is not an open book exam.

How's class seniority decided? Age?
Are sim partners assigned my seniority?

Seniority is based on age. I think they take experience into account when assigning sim partners.

Do they try to spread it out? Have a 121 guy with a CFI or keep CFI with another CFI , 121 with another 121? If that makes sense.

Years ago when I went through initial, and every time I've changed equipment sim partners were chosen by bidding. Experience etc had nothing to do with it. They have a number of sim schedules and you go bid based on seniority.