How do I send a campaign from a contact manager if one is assigned?

Using dynamic fields, you can set up the email defaults as coming from the contact manager if one exists, or fall through to the defaults

Default from name

%CONTACT.CLAIMED_BY.NAME[My Business Name]%

Default from address

%CONTACT.CLAIMED_BY.EMAIL.EMAIL_ADDRESS[some_other@domain.com]%

You can use the same concepts within your email body as well.