Can first trimester dating ultrasounds be a month off gestational age? (if taken at 8 weeks) could I have been 13 weeks and they measured 8?

No - off by a week. First trimester ultrasound should not be off by more than one week from the actual gestational age (due date).
Unlikely. Ultrasound dating is very accurate when done around 8-11 weeks of gestation. But, as always, life is not perfect and mistakes can happen. A woman should be able remember what she was doing 6 weeks ago (around the time of conception) if the u/s scan is showing 8 weeks gestation.