you can use the house wrap to hold the insulation back up(words to you). what is there is more of a blanket that means a real pain to return to origional condition. the easiest way to repair would be too build a box around the ducts/water pipes and insulate with r20 and then house wrap to keep the critters out. it is alot of lying on your back and avoiding falling insulation bits in your eyes. so you have to weigh the benefit from sealing all the joins on the main trunk with pulling down the insulation blanket and redoing the insulation.
if you can measure where the duct takeoffs are , you can cut small access holes and close up again with sheathing tape.