Supporting the border wall from one's own side

That is not true. Of course, there are L-angles where the short part points to the other side, that is, away from the load. We are also lower than the neighbor and had to support it; he doesn't want anything protruding onto his property, so we needed the special L-angles where the short part points to us. That is not true, they exist with the short part pointing to you and still looking nicer, you don’t see any reinforcement, magic word: "visible side inside". Just google L-angle or support angle visible side inside. We have the ones from Ehl.

The neighbor allows us to tear down the old wall and replace it with L-blocks + fence. The L-blocks may also face towards him. But we have to bear the costs ourselves. He will not contribute.