How to setup PO PDF to supplier?

Summary:

Q1: Should the contacts be always assigned to an address associated with site? for Contacts to populate on Non Catalog Request selection?
Q2: How can a requestor override the email address defaulted from a Contact?
Q3: what is the defaulting logic for email addresses to send PO PDF to supplier ?
Q4: if no contact was selected, can we send it to a specific email address during Non Catalog Request stage?
Q5: Does "Communication Method" has to be always set as "yes" at site level?
       If so, in what situation would this email address take priority?

Content (please ensure you mask any confidential information):