which configuration you want to change? wireless username and password? it has a web management page you can configure it. http://www.tp-link.com/us/faq-87.html

not following you,sorry. do you mean without extenders C2600 works fine?

it seems that C9 doesn't support dfs so band 2 or 3 are not open. But usually most devices support band 1 so it is enough for daily use.

do you bridge two extender together? it is not suggested to do that. But range extenders can connect to main router directly. like this: main router (wifi) extender 1 (wifi) extender 2 bad extender 1...

no heard of that, but you can disconnect other clients and keep just one then try again.

i think Build 20161025 is newer then the one 161018, so we can't upgrade it.