Is the planned construction of a garage on the property boundary allowed?

Is it allowed to build a garage that according to the sketch covers more than 70% of the small plot area?

The secret development plan could provide clarification.

Or are there any reasons that could still stand in the way of building this garage?

A last straw came to mind that you could cling to. I don't know the history of the creation and the then and current ownership of the parcels, but one could still argue a violation of the duty of consideration (§15 Building Use Ordinance), because according to your statements it would have been easy for the developer to build the garage directly next to yours in line. However, enforcing this legally could become difficult since the developer does not need a permit, and you would have to initiate a civil lawsuit. Maybe in a personal conversation with an authorized employee, you could appeal to their reason with this quote.