Time for some theorycrafting!
I think it's Ichika helping Miku stealthily throughout the chapter. Consider the following:
The lingerie that Miku got was probably from when Itsuki said she was going to throw it out from the previous chapter. She could have said something like "Instead of throwing it out, let me have it."
The bag of bread (and the lingerie) was probably in the bag that Ichika had on her during the last panels in the previous chapter. None of the other sisters had a bag on them from what I could tell. I think Ichika picked it up since Itsuki ran after Miku, Yotsuba went with Fuutarou, and Nino was preoccupied with wanting to get down from the mountain since a crowd was forming. The "My stomach hurts" would also make sense, since she could ditch group D to follow Miku and Fuutarou.
This one might be a total stretch, but for the pond thing, maybe Ichika dressed up as someone else and bumped into Fuutarou or pushed him which caused him to knock Miku over. I really have no evidence on this one but Ichika totally strikes me as the type to do the "push one of them so that he/she falls on top of him/her" scenario.