What is the ruling on what transpired between the questioner and her husband, and is it considered a lawful seclusion (khalwa shar'iyya), given the differing fatwas and jurisprudential opinions? And is the questioner sinful if she acts upon a fatwa that does not consider what happened to be a seclusion, and what are the implications of that?

The seclusion (khalwa) between the spouses described is not considered a valid seclusion, because sexual intercourse is not possible with it, and because the place is not safe from others seeing them without their permission. This is what the Hanafis have stated. Furthermore, there is no blame upon you both for taking the opinion of the trusted shaykh who issued a fatwa to you that one divorce occurred, so the marital bond between you remains, and there is no need for worry. You should repel obsessive thoughts and avoid frequently seeking opinions from different muftis.