Workflow based on multi choice list

I would not recommend doing this in the worklfow. Do it in the InfoPath form with OnChange event's attached to the fields that need to filled out with logic checking to ensure they are not blank. Doing it in the workflow would require the workflow to be run when the item changes and that will require you to code the workflow to check (I'm going to guess) a whole bunch of other business logic conditions. Also change based workflow tend to trigger a lot and you'll see that in your logs behind the scenes.

If you are going to do this in workflow - break down the different form areas into Custom Tasks - get all the data via the task (customize the form for the task) in there and then after the task is marked as completed write the complete set of data back to the form.