If DMZ works, maybe ports you forwarded are not quite correct. Based on this assumption, it deserves to try DMZ. You can also try disable hardware NAT. If it won't help, you can enable it again.

1. Can I use BOTH my standard AV500 powerline units AND the powerline wifi extender at the same time and all on the same POWERLINE network, I can get all three untis to have their POWERLINE connected...

The modem is in compatibility list. Maybe TP-LINK has a beta firmware or some other methods for you to try. http://ticket.tp-link.com/index.php?/Tickets/Submit

That's right according to my understanding.

It is good that you can connect 15 devices. I can only connect 10 devices at the same time by a same level router. :(

Replace it under warranty or contact TP-LINK directly. http://ticket.tp-link.com/index.php?/Tickets/Submit