192.168.200.250 is a private IP... It cannot be accessed remotely. On the modem, open port 80(you'd better use a different port like 81) for the router to remotely access it with http://modem's WAN...

TP-LINK firmware has no such a feature... You may provide the instruction to your customers about how to change SSID and password. It is quite simple...;)

What's the firmware version of your WA830RE? Ensure your main router has no MAC filter rules enabled...

Check the line rate(upstream&downstream) on status page of W8980 and on another router's web interface. Post screenshots here.

Did you try to disable anti-virus software and firewall and check if the problem continues?

Check if WLAN Autoconfig is enabled or not. http://www.tp-link.com/en/faq-306.html