Is it possible to adopt the Hanafi madhhab in atoning for previous 'umrah circumambulations with a sacrifice (dam), given the ignorance of the ruling and not remembering having performed ablution at the time?

Scholars differed regarding the requirement of purity for circumambulation (tawaf). The majority stipulate it, and this is the issued fatwa. Some scholars consider it an obligation but not a condition, while others deem it desirable, which is the choice of Shaykh al-Islam Ibn Taymiyyah and Ibn 'Uthaymeen. There is no blame upon you for adopting the view that purity for tawaf is a Sunnah, as it is a recognized opinion. Indeed, some jurists have explicitly stated the permissibility of adopting the weaker opinion after the act has occurred and it is difficult to rectify. There is no blame upon you for adopting the view that purity is not a condition for tawaf. However, if you were to sacrifice a sheep in the Haram for each 'Umrah and distribute it among its poor, or appoint someone to do so on your behalf, that would be better.