It is NOT impossible, thanks to some scripting and a couple of free services.
hostname=$matichost&myip=$current IP&wildcard\ =NOCHG&mx=NOCHG&backmx=NOCHG\"\r\ \n\r\ \n:if ($current IP !
I'm afraid that you're going to have to approach this from the client end.
Changing the timeouts is only going to help clients the next time they check in, which is usually after half the existing lease time, or sometimes during boot.
This user account only needs limited privileges, it must be a member of DNSAdmins, DNSUpdate Proxy, DHCP Users and DHCP Administrators.
While you are on this tab, its worth setting the conflict resolution attempts to 3 or 4 which gives DHCP the opportunity to resolve any issues itself.