1) The purpose for suexec is to make your website safer and not the pother way round. So you shall of course use suexec.
2) Use the password cahnger plugin for roundcube.
3) Today the traffic is normally not limited anymore by hosters as almost all hosting offers include all traffic. ISPConfig 126.96.36.199 contains traffic stats per site, if you want to suspend sites based in this stats you can e.g. write a script for that.