You are right about that.
I couldn’t have said it better myself; the deviation from your rough principle sketch is quite significant (as I had suspected). Now one can see much more clearly and develop well-founded proposals.
Even better, you also upload another section (radius about 100 m), then one can also recognize "actual building boundaries." If possible, choose a map in which the single-story outbuildings are differentiated.
The current house No. 14 seems to slightly overbuild its plot boundary to the western neighbor. And from the plot numbers, the suspicion is confirmed that plot 15/2 was separated/demarked from a former overall plot 15.
I would—of course as an "expression of opinion, not legal advice"—consider departing significantly from the path of the measures you described. So let’s assume you were someone who listened to Charly’s father’s advice, then the following situation or approach would be obvious: 1. presumably, a merger of "15 now" and "15/2" would result in nothing other than the restoration of the overall plot "15 old"; 2. all the players are (through marriage) related to each other; 3. your father-in-law (= the biological testator of your wife) is involved as an owner in this entirety, namely in part 1 with house A together with his siblings in equal shares and in part 2 with house B jointly with his wife (who is also the biological testator of your wife).
Whether someone is a rogue or a fool remains open and is up for choice. Honestly: Charly’s father—and I understand him very well here—considers the measures of the new subdivision of the property(ies) to be a somewhat unnecessary middle-class nonsense that creates unnecessary fee obligations and tax events (in the event of inheritance even a second time). From his point of view, the following is just obvious: 1. contribution of the previous ownership shares in the plots "15 now" and "15/2" to a property community; 2. leasehold contracts of the property community for the "blue framed" area to the spouses "You & wife" and for the "red framed" area to the parents-in-law; 3. settlement of the father-in-law with his siblings not through land, but through shares in the community; 4. furthermore, much more favorable gift possibilities arise from the parents-in-law to your wife and you, also via the transfer of shares.
In your leasehold contract (2./blue), handling of the costs for clearing the area by demolishing house A is also arranged. At least Charly’s father claims, and I also know suitable lawyers and tax advisors. But with that, we are already almost at the end of my honor-free thinking aloud.
Are houses A and B actually in such different conditions because the parents-in-law still occupy B, but you want to demolish A?