Is KfW funding with repayment grant possible for me?

If that were the case, and I buy the one apartment from the one owner, would I still be a first-time buyer and be able to claim the KfW subsidy?

Who is the developer? If it is, as writes, e.g. a family/heirs' community, then you are clearly a first-time buyer. It is still an unusual story (like, for example, with a developer community, where everyone basically builds one apartment).

Who is the builder? Is it, as writes, e.g. a family/heirs' community, you are clearly the first buyer.

It is still an unusual story (like e.g. with a building owners' association, where everyone basically builds an apartment).

The builder is a GmbH that sells, builds, rents, etc. apartments/houses.

It is also strange that in the building application from 2014 a different builder is registered than the one currently indicated on the construction site on the red dot.

Unfortunately, I am not familiar with KFW funding.

The KfW is based on the Energieeinsparverordnung.
Thus, there is no KfW55 apartment, only a KfW55 house analogous to the energy certificate.
See FAQ of the KfW.

The apartment is off the table for me. The original builder backed out and then the builder changed again. The current builder is somewhat dubious, a GmbH founded in 2017 with little capital, the managing director has also changed, is not reachable by phone, and the names of the people involved do not exactly inspire confidence. Then there is the issue with the subdivision which is strange (possibly investors). I'd rather stay away from it; in the event of later problems, you probably won't be able to reach anyone anymore.