When applying to grad school, I did not think enough about the requirements of disclosing past education. I failed to disclose a school I attended because it was so short and brief and I received all W's, there is no GPA, but there is a transcript. I did not realize how big a mistake this was at the time. Now, I have been notified of acceptance from the department, still waiting official acceptance from the institute, and I am very nervous that this will all get messed up. What should I do? Tell the school? Decline offer and reapply next year with correct info?
Your options seem to be: tell them, don't tell them and hope for the best, or withdraw.
If you tell them, the worst that can happen is they revoke your admission and invite you to reapply next year. If you withdraw, this will happen for sure. So, I wouldn't even consider withdrawing.
I usually suggest not over-analyzing things, but in this case, I wouldn't just let the sleeping dog lie. You don't want this hanging over your head for the rest of your career. So, I would just send a brief e-mail saying exactly what you did above (and just as concisely) and asking how to proceed. I expect they will say that they will update your record and do not anticipate any problems -- but you should prepare for the worst-case scenario just in case.
Given that grades weren't assigned, there is no way to evaluate your work based on the transcript. I doubt that it matters much whether you inform them or not. Full transparency would suggest that you do, of course.
But if all the grades are W, meaning "withdrew", you might need to explain why.
They have judged that your supplied material is sufficient to decide you a candidate for success. I don't see how the additional information can change that. The same would be true of any informal self-learning you engaged in.
But, IMO, it is a pure judgement call. If you don't tell them and are called on it, your explanation here would likely be sufficient. If you do tell them it would be a bit of embarrassment, but not a deal killer.