I don't think there would be any problem with switching the couch with the chairs. You should also be able to leave them in place when the slide is brought in.
As for the treadmill, you could put a nice piece of laminate flooring down on top your carpet to set the treadmill on and that would spread out the extra weight when having the treadmill with you. Bryan could give you a "for sure answer"
You will have some shaking while using the treadmill. You can put some extra stabilizers on to help lessen that. My wife says she can fell me walk in our WC and I'm 300 lbs. of BS. I thought I walked lightly.