Displayed KFW conditions

The plan was actually to build according to the [Energieeinsparverordnung]. The additional effort for [KFW55] should not be that great and in terms of costs would be around maybe 10,000€. The interest saved as a result would be higher.

Yes, the bank would only sell to me if I complete the full financing with them.
Who the bank sells the property to is probably up to you; in case of doubt, they will wait for the next customer who completes the full financing with them.
Thread: [Grundsätzliche Fragen zur Finanzierung]

That is an outrage from your bank. But never mind. Regarding the KFW the following information: normally you get a small discount on the interest. Usually, a tenth is deducted. I have never heard of an additional charge being added!

You always pay the nominal interest rate as the loan interest. The effective rate only shows the payment frequency (monthly / quarterly) or fees. The land charge of the main bank is usually registered in the amount of the KfW loan. With this sentence, KfW wants to distance itself from all the smart alecks who then come up with the costs of the land charge and claim that this was not taken into account in the effective interest rate.

As described in another thread, my main bank only offers me an interest rate for the classic annuity loan with an interest surcharge of 1% above the market rates. Unfortunately, I am bound to this if I want to have the property. Therefore, the KFW interest rate would be considered favorable. My concern is therefore that the bank will charge 2% for the KFW loan instead of 1.45%.

Yes, they can theoretically do that. They say, yeah, you can get KfW but only at 2% or not at all. A bank is neither obligated to offer KfW loans nor do they usually offer more than the standard (home ownership). I know your case from the other thread. Anyway, I wouldn't buy anything there.