DHCP server is disabled on W9980, isn't it? Then it is not working as a router actually. Anyway, I should say, you'd better use W9980 as a router with DHCP enabled and Disable "Allow Guests to access...

System Log is under System Tools. Under DHCP, you can see DHCP clients list. What are you trying to look for?

If both your new D9 and previous router dropped ability to WOL over internet after about 10 minutes, is it a network adapter issue? Is it possible for you to test by a different computer?

would that allow someone in the guest network to access all the PCs/Devices on the "private" network? Yes. To avoid it, you'd better use W9980 as a router instead of an access point and Disable...

D7 has no such a feature. It can be configured as either a wireless router or a wireless modem router.

WDS bridging? Did you disable DHCP on MR3220? http://www.tp-link.com/en/faq-227.html Your MR3220 is V1 or V2 indeed?