Property Purchase - Questions about the Process

Is a purchase already possible on this basis

No, but a notarized purchase contract is.

Can the Baulast be registered now at all?

No. Baulasten require a concrete building project. There are exceptions, but in this case IMHO they do not apply.

If they refuse the Baulast

The bank cannot prevent the Baulast, at least not directly. Of course, it can refuse a financing commitment if its rules are not followed. Since the Baulast is a public-law obligation, the bank's involvement is limited to the fact that the Baulast affects the value of the property.
It is quite different with easements, usufruct rights, and limited personal easements. These are (generally) civil law agreements entered in the land register, which the contracting parties can freely negotiate among themselves.

So it is not yet clear whether it has to be a 0.5m or 1m setback Baulast (roughly speaking).

But that has nothing to do with the missing survey. You have to agree before the survey what is to be measured. If in the end one or two square meters more or less are entered in the land register, that has technical reasons and should not be due to imprecise wording with a competent notary.

If it then turns out differently, so be it.

No, then it is incompetence, sluggishness, or fraud.

But this is not what it is about here, rather about whether and how the legal processing can be carried out.

One concludes a notarized purchase contract in front of the notary, which is then executed step by step. If one step cannot be carried out because, for example, another heir appears, the further steps are not executed either, e.g., payment of the purchase price. You should only clarify in advance who then pays the notary fees.