Energy expert refuses further support

If I were not a directly interested party myself, I would probably also tend to classify it as a new building without prior knowledge. But that is not the case! The building authority has investigated very thoroughly and internally coordinated for weeks. There are various reasons that led to the decision

You are dealing here in the forum with people who do not simply make such assessments lightly, and that probably applies to the energy consultant as well. However, he will not spend weeks playing through internal approvals – this probably sufficiently explains why there could be different assessments from the building authority and the energy consultant without ill will. Moreover, I had already explained to you where a high "hypersensitivity" of the energy consultant regarding the risk of penalties in case of possible fraud comes from. "All's well that ends well," I still maintain that the energy consultant acted understandably and that the matter could also have been clarified with him.

Understanding also that he got in touch a whole 6 months after the confirmation of the application (BZA) and after I had signed all the contracts and dismantled the existing house?

Yes. On the desks of energy consultants it's "landunter", so he may very well have just recently gotten around to addressing it again