Without seeing the form I will take a shot with what I would do (assuming i understand correctly).
I assume that a complex form would be flowable which means when you hide and show objects, the other objects move.
For this to work, you will be working with the pre-print and post print events.
I would have a row for the combo and a row for the checkboxes. The checkboxes would normally be hidden when viewing on screen. The button is just for my example, you use the pre print and post print events for the form.
this.resolveNode("Table1.Row2").presence = "visible"; //checkboxes
this.resolveNode("Table1.Row1").presence = "hidden"; //dropdown
this.resolveNode("Table1.Row2").presence = "hidden"; //checkboxes
this.resolveNode("Table1.Row1").presence = "visible"; //dropdown
Before Clicking Print
After Clicking Print
After printing or cancelling the print job
Pre and Post print is very handy for customising your printouts. By using a design like this example, the combo row is basically replaced at print time with the checkboxes thus not affecting the layout or length of the form.
NOTE: If you have a goal of filling in online and then printing, i would make code so that when items are selected in the combo, it also checks the boxes. Even though they are hidden until printing, they will still get checked while hidden. Simple 'if' logic can achieve this.