Point 1 was the problem, that on the OpenVPN client computer exist two networks and network cards:
a) Traditional LAN adapter (192.168.199.99)
b) OpenVPN LAN adapter (10.10.10.5)
But I was able to solve that with a client specific setting "Tunnel network" = 192.168.199.252/30. Now packets originating from this machine appear as 192.168.199.254, and that is okay for me.
Point 2 is solved, too: I feared that the user at customers OpenVPN machine could be able to change his IP address to something else to get access to other networks, but if he does so, he'll never get answer- packets back from the server.
Then I have everything!!! Thanks a lot to you dreamslacker, and everyone else who wanted to help on this topic, too.
Hugo