if you work in law enforcment, then it's no problem to contact the ISP and ask for the addy in questions email log, they have to backup everything by law, as you already know.
as for the chat log, if it's been deleted, then your high tech crime unit will be able to recover deleted data.
kinda makes me wonder why you would be asking on a internet tech site when you have so many resources in the police department at your disposal.