Is the wife's condition stipulated to her husband, that he permit her to attend weekly scholarly gatherings, invalid according to other schools of thought besides the Hanbalis?

The condition you stipulated to your husband before marriage regarding your gatherings with your sisters for studying knowledge must be fulfilled by him according to the Hanbalis. This is because they obligate the husband to fulfill every condition that brings benefit and advantage to his wife. This is what Sheikh Ibn Uthaymeen preferred. Therefore, your husband is not permitted to violate this condition or prevent you from going to study Islamic knowledge. Furthermore, being lenient with the wife in this matter is among the noble morals and good companionship, as long as no religious harm results from it.