Given an EmailMessage object, you can just call the CreateForwareMessage() method:
var forwareMessage = item.CreateForward();
Regarding the other question: Get the MIME content of the mail and attach it to a new message:
item.Load(new PropertySet(BasePropertySet.IdOnly, ItemSchema.MimeContent));
var mail = new EmailMessage(service);
var attachment = mail.Attachments.AddFileAttachment("Original message.eml", item.MimeContent.Content);
attachment.ContentType = string.Format("message/rfc822; charset={0}", item.MimeContent.CharacterSet);
mail.ToRecipients.Add("hkrause@infinitec.de");
mail.Subject = "testmail";
mail.SendAndSaveCopy();
EDIT:
Create forward message and set reply to header:
var fw = item.CreateForward();
var fwMsg = fw.Save(WellKnownFolderName.Drafts);
fwMsg.ReplyTo.Add("personA@company.com");
fwMsg.SendAndSaveCopy();