I think if you're going to be returning to pumping when you return to work that it would be best if you continued to pump at some point during the day. I know when I've had breaks from pumping (not that I ever did it much and don't now) that it's been difficult for me, if not my body, to adjust to pumping again.
As for the formula supplement, I'm not sure. If you think you're still going to need to give it when you go back to work, it might be worth keeping it just so you're not potentially in a situation where DS has gone off the taste in the 2.5m you're off.