A potential lawsuit against my construction project is looming! What now?

I wouldn't worry too much about this for now.. Who is even claiming that? Neighbors usually only need to be involved if there are legal deviations, you can simply check that in your [BauOrdnung]. If it really is a building plot and you are building something within the legal framework, no one can say anything to you... Of course, it's still not nice to have such people at your door - they probably won't be your best friends.

A covert client model?

Administrative acts without legal remedy instructions can indeed be contested for one year. To what extent a building permit with legal remedy instructions overrides this, I cannot assess - but even in the case of such a building permit with legal remedy instructions, your neighbors could still file a complaint for four weeks.

I have understood it somewhat that way as well. And to me, it also came across from the authorities that one has put oneself in an uncomfortable situation. Through precisely the omission of informing the neighbors. As far as I remember, a lawsuit would first be reviewed by the legal committee, which would then either reject or approve it. But then one would be incriminating oneself. The whole thing is indeed annoying and it is not a good basis for a neighborhood. Nevertheless, I want clarity here and want to act in advance instead of reacting later.

Legal committee? You file lawsuits in court. If it ever comes to that, I would invest some money in a lawyer who will explain and break it all down for you.

Apart from the hidden developer constellation as it sounds to me, and with the warning that a) I am not a lawyer but b) such a person should be consulted here:

As I see it, your start of construction is pending as long as the neighbors who have become eligible to file objections longer due to the official default all and finally (declared in writing at the competent court) have stated that they do not want to sue. If even one of them does not agree, I do not see the deadline being shorten able – the default itself cannot be remedied.

As long as the neighbors have only declared their non-litigation to you or the developer – even if they were to do so all together! – in my view the deadline must be awaited.

Your lawyer will explain to you the waiver of appeal by those entitled to sue.

Knock on the door of your new neighbors, maybe the old owner was just an idiot, that would also suggest hiding it. Especially if you get into conversation with the old neighbors exactly because of this idiot, you have an advantage.