How much good neighborhood is worth to someone is something everyone has to decide for themselves. I would ask a developer how much of your house they would build "for free" if you give them half of your land in return. That could be the cheapest way to get a house. If you also have an acquaintance who buys the other house from the developer, then you would also have good neighbors.
Not a good idea. The developer buys the land for sum X and sells it on to their own friends with a hefty markup.
It is better to optimize paths and costs through simultaneous construction start and material procurement; I would definitely transfer the land to friends/acquaintances beforehand, especially since otherwise notary fees and property transfer tax would be incurred twice – and who pays that? Certainly not the developer ...
Best regards Dirk Grafe