Roy,
I’m currently at York starting week 7 of the old TCB course. I have no idea if the new course is easier or harder.
With regards to pre-reads, you will need to know if you are on an TCB or AB course and I would do no more than learn the phonetic alphabet if you don’t know it & key definitions and principles in chapters 1 & 2 in the relevant TS2 For TSB or TS 3 for AB anything more will just confuse you if you don’t have a railway background.
As for failing I had the same fears but believe me you will be supported. No one on our course had got lower than 95% in either week 3 or 6 exams, yes there is a lot to learn but as long as you put the extra work in at night you should be fine, as my LOM told me, He wouldn’t have employed me if he wasn’t certain that I could pass the course.
Even if you did struggle and failed you would at least get another go at the course.