Already removed tree worth preserving in the development plan

I don't want to prevent the tree. On the contrary, we want to have a tree there.

Yes. And there are different kinds of authorities. Some don't care, as long as you plant a tree. Others give you exact specifications, like a linden tree with at least 10 cm trunk diameter at planting, because there used to be a linden tree there. And then you don't want the linden 5 meters next to the house, because it grows big. I also see little reason to tie myself to that problem as a buyer.

So make sure to take precautions in any case....a friend of ours pruned two chestnut trees he had in front of his house that were casting shade. He was reported, and the city has a pretty strict tree ordinance. Since chestnut trees cannot tolerate heavy cutting back, the act was treated like a felling. A 6000 € fine and three replacement plantings with conditions. However, these new trees were also allowed to be planted in a friend's garden.

I would definitely not talk to any authority about the missing tree. Possibly with a lawyer friend (and then take a photo). There is no obligation to have a tree in a specific spot, only the prohibition to remove an existing tree without permission. And the person who removes it has to take responsibility (if anyone notices), and then they are required to plant two new trees somewhere. Just plant your little apple trees and that's that.

Thank you very much for the many helpful suggestions! They helped me a lot, and I think I now know how we will proceed. :)

So you don't have to worry about getting into trouble because someone thinks you might have removed the tree... Such things can be proven either with a photo at handover or, if necessary, easily via G.. Earth or the Office for Land Management or similar within a very short time. You can find out which deciduous trees should generally be planted from the responsible office, provided it is not clearly stated in the development plan. So I wouldn't expect any problems for you. You can't conjure a tree back that should have been there, so no office will demand anything in that direction.

It helped me a lot and I think I now know how we will proceed

And it would be nice if one briefly explained how they want to proceed here, especially when they have already asked for advice and received many good answers.