Matt, i feel very sorry for what's going in past 24 hours, your site is currently under huge DDOS attack from UGNazi's, allthough you are senior and have more knowledge, but i would say take whmcs site offline, close all the accounts for next 24-48 hours, pack you stuff and move to a new datacenter DIRECTLY, not through a host, DIRECTLY buy a multi server infrastructure with DDOS protections enabled and lots of security, unpack yourself there and say hi to everybody, !
it's going to be definitely hard time keeping up at this stage, what if the payments of new clients signng up for whmcs are at risk, what if the gateways have a new email id to send the payments to ? well there's a lot to inspect and seriosuly, go down offline for some hours and then come as a strong man with security.
Good luck
We are with you !