The charities focused on autism usually have employment related advice and support, they might be able to provide some careers advice and help you identify a better path for you.
I work in HR and have a legal background. My advice is to resign, given the information you have provided your current employment is not going to continue - the drug driving is really serious and not telling them exacerbates that.
If it was the exams I think you would have lost the training contract - failing one or even being borderline on a second exam they might have given you a chance but the whole lot means that they cannot invest in you any more - it’s too much of a risk for an uncertain outcome. I can’t see why they would give you a settlement agreement at all - we only use them for exits where there’s a risk of litigation that is difficult to defend, and I can’t see that here.
I would advise you to really think carefully about what you are capable of doing and what your strengths are. You will need to consider the criminal history if you are charged and/or convicted as it will exclude you from some careers or organisations.