you could create a rule that would switch views to a read only format. But since you need someone else to modify the document after it was submitted then you could check to see if the current user is the same as the person that submitted it and then show a message stating that they can not edit it and then close the form. However with that way everyone else that didn't submit the form could edit it. You could extend that and take the users that are the approvers and create a rule that if form is submitted then only those people can edit, all others get a message and close. Also could use checkout feature.
there might be better ways, others I am sure will post up how.