Is a new construction possible with our cost estimates?

Can one already fictitiously assume in the preliminary building inquiry that our plots A and B already belong together, or must a merger have taken place beforehand?

It is all "just" an inquiry. There you can and must work with assumptions.

Here is the response from the building authority:

Without a merger of the two plots, parcel B becomes an outer area reserved exclusively for agriculture.
New buildings in the outer area are not permitted without this privilege.

If the plots are merged, it might be possible to assign the garden usage structures possibly to the inner area because there would then be no separating boundary.
If classified as inner area, §34 of the Building Code must be observed. Here you can find the 5 criteria for integration.

I would not go to a general contractor for this, but to an architect who is familiar with complicated building law situations. It is exactly the case that building is not allowed in the outer area, and also merging at this border usually does not change anything. Maybe the local building authority is more lenient, but the outer area is sacrosanct in Germany.

but rather to an architect who is familiar with complicated building law situations.

Yes, I forgot to mention that. Of course, you can do it on your own. Request a site plan and then mark it in as logically suggested and as you have thought it through. However, an architect has a more professional way of thinking and knows what matters. For example, there is the statement of Paragraph 34... but these are often like Bohemian villages and how the building authority wants it, so not necessarily feasible for a layperson. Besides, he knows the regulars at the building authority. He might even have network contacts there. For a small flat fee, he takes care of it. You should reinvest a few hundred euros to get a positive preliminary decision. Don't end up including as much as possible from the agricultural area out of fantasy and thereby falling under Paragraph 35. Then nothing will work anymore (except agricultural operations).

And never rely on emails or verbal statements from the building authority. Their "could and should" cost us 1.5 years of waiting for our building permit for construction in the outer area. Sometimes, recommendations and theses previously made by the building authority were withdrawn again in the next conversation. Only the official decision really counts. That’s why I recommend hiring an architect. Without one, we would have probably waited 3 years.

if one of the 2 plots is to be made into one, then all houses located and planned on it belong to the landowner, i.e. your parents. Only they can take out a loan and register a land charge. After completion, dividing the plots again is probably not possible. A building inquiry can already be made now, but before spending more money I would secure myself as to what possibilities exist. existing house given as a gift, GBR, GmbH etc.

if two plots are to be combined into one, then all existing and planned houses on it belong to the landowner, i.e. your parents. Only they can take out a loan and register a land charge.

No, the children can also take out the loan and initiate registration of a land charge (however, the parents must be involved in the land charge agreement since they are the owners). The parents are then not personally liable, but only liable in rem with the property. Whether this is sensible is another matter.