Our plan for the new building - What next?

Stay calm for now. It’s definitely already a very good idea of yours to seek help here. You get that in this forum for free, honestly, and directly.

Please don’t let the deadline for the Baukindergeld stress you, but rather assume at first that you won’t get it. An average planning phase takes about 1 year. You have to take that time because everything else is based on the planning...

Building is a matter of trust. If you don’t trust the architect or the chemistry isn’t right, it was a good decision not to continue working together here.

Your feeling is right here, too! Under no circumstances sign anything here. He is deliberately putting pressure on you to close a deal.

Your feeling is what you should rely on at the beginning, especially when you are a complete layman at first. So trust your feeling and make a clear demand to the prefab house manufacturer: either detailed floor plan planning and then signature or goodbye.

Not necessarily. There are plenty of big and small providers who do the lion’s share of the preliminary planning for free. That was the case with us. But it is also important that you invest enough brainpower yourself.
Also, the forum can help you if you want to have a floor plan discussed objectively.

If I were you, I would first take a breather, break off contact with the prefab house manufacturer, and quietly pick out smaller local general contractors (solid construction and timber frame) as well as some larger single-family house manufacturers with a good reputation and start over again.

Keep your chin up!

That is good, and you also won't get far with a "dummy," which really makes no sense for a formal building application.

My lawyer says I shouldn't write from which of my buddies the visit would actually come.

There is no "normal" procedure anyway because of building in the (in-law) parent’s garden. But still, you are not an isolated case, and many like you have already been helped here in large numbers.
1. Fill out the questionnaire at the top of the floor plan section
2. attach a site plan etc. of the property
3. bring the DIY floor plan, gladly even on scribble paper.
Then it will become clear which professionals you need on the way forward.

Oh, and regarding building on a "foreign" property, there is a thread here that you might want to take another look at.
Sorry if I cause even more confusion with this, but the kid is not in the well yet...

https://www.hausbau-forum.de/threads/haus-auf-Grundstück-der-eltern-erbschaftsprobleme.33972/

The bottom line is: The house would not be yours, even if you pay for it.

I estimate that I have probably already spent 100 hours tinkering with some floor plans...

I never dared in the past few months because most plans here already had proper dimensions, etc., and so far we have only played around with Roomsketcher

We are simply laymen,

The issue gets even more complicated because the plot is a leasehold property of the church, but I thought that would totally go beyond the scope here... We had basically asked the church once whether we would be allowed to build a second house on it. In principle, there is nothing against it, but of course you need to have a concrete plan to say yes or no...