What does the property look like anyway? Are you sure you need L-blocks? If yes, then it becomes critical anyway. Assuming the curb sits on the property line, you also need a base and a small foundation for your L-blocks. So you’d have to set back 10-15 cm. It looks absolutely ridiculous in the end if there’s a 10 cm green strip between the curbs.
Not really, they sit in the concrete bed without overhang and have additional support through the load.
Of course, the neighbor has to remove the encroachment. He could even build against your L-blocks. It will still annoy him, although I cannot imagine that he wasn’t aware of the encroachment. You’d have to be very blind, and the executing craftsman also knows very well that you cannot do it that way.