Forget the workflow for a moment.
The design of the form is pretty easy to achieve, each approver would have a view of the information at a particular stage of the process.
You could use form load rules to open the form at the correct view, when combined with a Form Status/Stage field,
You could have fields set to read only as required. Each person, technically submits the form to the library, only on the last stage would you have 'form sign off'.
The workflow does the email routing, permission setting etc.