Based on vent pipes seen in the roof in aerial photos, I suspect the bathroom was at the right rear of the house. Plus the rear photo shows smaller windows at that corner room.
Based on the time of night, one would expect Jayme was in bed fast asleep. From my own child, I know it sometimes it takes a lot to wake them at that age. Assuming James was shot first and then Denise ran, why would she run to a bathroom? If the location is on the rear half of the house then it's a second floor room. Plus, probably smaller windows to escape through. It's rare any interior door in a house is going to hold up to much so it's not a secure room.
I'm thinking maybe Jayme was still up and taking a bath. Perhaps they'd been out quite a while that day (we know about the family gathering and those can drag on). And we know James worked, so it's easy to imagine Jayme wanting to stay up to tell him something. Or perhaps she was running late with school work. Anyway, if Jayme was taking a bath (school the next morning!) it would explain Denise running to that room. Otherwise why not run to the door to the deck?
Of course it may have just been an all out struggle and she was clawing her way wherever she could.
Currently, I'm thinking somebody got off the evening shift at Jennie-O, went home for a gun, and went to the Closs house in anger. Perhaps they were made at both parents but it didn't extend to the child so they took her with them to decide later. We can see all sorts of ways co-workers could get mad, especially when a couple work at the same place. There's always jealousy, flirting, favoritism, firings, demotions, suspensions, accusations, etc.