The solution will be what you have suggested:
$WA_MailObject->ReplyTo = "".$row_rsMailSettings['replyto'] ."";
$WA_MailObject->ReturnPath = "".$row_rsMailSettings['returnpath'] ."";
This does not work. When I check the email in outlook on both accounts it defaults to the senders email address...which is another issue. If I choose a dynamic value for the senders email, it does not work...hence I have hard code the address and then, like I say, the addtional headers default to the hard code email address.