Can you run the furnace while the slide is pulled in?

25 degrees for a few hours--it will not freeze. Water with stuff in it has a lower freezing point that clean water. Add some salt, sugar, or antifreeze and lower the freezing point even more.

AS for running the furnace I would not, but my slide covers two of four vents. You need to decide if the vent covered by your slide will heat the slide bottom enough to cause a problem. No one else knows how hot your vents get or how tight the slide is to the vent.