My son had to do two courses of Roaccutane (called Accutane in the US here).
So, I only know it from the perspective of a teen male taking it, so won't be able to help with the PCOS factor....however...
Like you, the first round cleared up his acne, but then months later, it reappeared. The dermatologist said that this happens with some people...that one course is not enough. He upped the dosage slightly on the second round and my son's acne finally cleared, pretty much for good (he does still get an occasional spot here and there, as he is 18).
Accutane (or Roaccutane) is derived from Vitamin A and so ingesting more Vitamin A on top of that could be toxic. The blood tests, which my son had also, are to determine if the body, especially the liver is processing this potent Vitamin A rich medication.
You are right about Roaccutane or Accutane causing horrific birth defects! DO NOT USE IT IF YOU ARE TTC!!!
The side effects for my son weren't too bad...a little bit of a sore back during the first course and very dry, chapped lips both times he took it (because from what I understood, the medication is shutting down production of the oil glands that cause the oily skin and acne).
The dermatologist tried other meds (antibiotics, and topical cremes and such) but Accutane was the only thing that finally got rid of my son's severe cystic acne.
So, perhaps you might need to take it again. But, please be careful if you do...it is NASTY stuff! So strong!

Kathryn