In response to your first question TCR, there are many people that don't get through the college - roughly 30-40% success rate at a guess.
After that the validation rates at units vary from 25-90% - only based on hearsay and what I've read on here.
IF you fail a written exam/oral board at the college you will get one more go, then if you fail again, you fail the course and go to a training analysis where they decide if you warrant a recourse. If not, you get a training review where you can 'argue your case'. In general if you fail a particular course twice you will no longer be employed - there are some exceptions to this but not many. However, you can fail one course, pass the second time, then fail the next course, get a recourse etc Some people, who want to stay with the company, get jobs as ATSA's or in admin.