How can I set the requester of an email ticket to reflect the user information shown in the body of a ticket created via email?
Unfortunately the email body is not used in any way to assist in identifying the requester of a ticket. Zendesk uses information from the email headers to determine ticket requester. Specifically, the main way a requester is set is by the reply-to: header flag. If the reply-to: header is not present, from: will be used in its place. This functionality is hardcoded into the system and unchangeable.
Agents can use mail api #requester syntax to set the requester on a ticket. Using the Mail API to update ticket properties from your inbox.
For more information on email threading, see Why are incoming emails threading to the wrong ticket? and Ticket threading issues.