As you mention, you will need SharePoint Workflow here. You do not want nor need multiple submits, nor do you need to email the entire form. My first question is - will the users in the email process (the "next person in the list") need to edit the InfoPath form and put something in it, or are you just routing for let's say approval? Also, SharePoint 2007 or SharePoint 2010? InfoPath 2007 or InfoPath 2010?
You may wish to use an Out of the Box SharePoint Approval workflow, if the users in the list are usually the same or similar. You would either manually kick it off for each InfoPath form, or kick it off upon creation of a new InfoPath form.
You may, instead, wish to use a SharePoint Designer workflow. Gives you much more flexibility as to who to send the emails to (you can get an approver out of a SharePoint list for example, gets a little muddy, but can be done). Designer workflows also allow you to change the email that gets sent rather than just stating that they have an approval task.
Sort of off topic, but a question you asked: In general, if you want to update a current form and not create a new one, create your OWN Submit button.
Create a field called FileName
Create a Form Opening rule, setting that file name to some concatanation of fields and words - example:
concat("Probation Form - ", userName(), " - ", now())
Make sure when you set the File Name you use the Now function in part of the name to capture a date and time - making a pretty much unique file name (can anyone submit 2 different forms in the same second?? :-)
Make sure that you have a CONDITION on setting the file name - only set the file name if it is Blank. (That way, it sets it the very first time the file was opened and not again)
Finally make a Submit Data connection, and use the FileName field as the name of the file. Make sure you check the checkbox to enable overwriting of the form.
Use that Submit action in the Rules of the button that you place on the form. Optional - close the form after submit.