Change upsets chickens but only for a couple of days. They show it by a glitch in the laying, but they will all start laying quickly again.
A couple of pieces of advice, take what you want:
It is very common for a 8-12 birds to use only one or two nests. You are short of space, so I would pull out two of the nests, or that long nest. I do like how you have your roosts set up. 3-4 hens could fit in there, but in that small of space, I would not keep any of the roosters. Roosters in small spaces can cause a lot of problems.
As for the new hens, it is a good sign that at least one of the new hens is laying. A laying hen is higher in social order than a pullet. Once they all start laying, they will probably all get along. I don't think you have a broody hen, they are all very young, and it is a rather unusual time of year. I am not saying it can't happen, I just don't think it is. I think you having a bird hang out in the nest due to space issues. They are in a hideout, until someone comes in and wants to lay there. Then they are forced out. They are using the nest as a hide out, give them some in the run.
Look at your run. Often times they are just an open rectangle. Put up some roosts in there, add some hideouts and blind spots. Just lean a piece of plywood or a pallet up against a wall. Put a pallet up on blocks so that birds can get on top of it or under it. This allows birds to get away from each other. Make sure that each hide out has two escapes so that a bird cannot get trapped. Set up a small piece of plywood in the middle of the run, and place a feed bowl behind, so that a bird can eat there out of sight of the others at another feed bowl. A full crop gives birds confidence to work on social issues.
When birds meet or come together say at feed bowl. The higher pecking order bird will peck at the lower bird. The lower bird will move away, acknowledging the social order, often times moving out of sight for just a moment. I have seen them do that, and seconds later come back out and eat beside the higher bird. In an open run, there is no way to get away from the higher bird. So it appears that the lower bird is challenging the social order, the higher bird must continually attack to establish the higher position. It can get ugly.
This is a fun hobby, but you have to accept that chicken society is a pecking order society. They will NOT just be nice. They need enough space, food and clean water. Then they will be happy and give you eggs and it will be a pleasant hobby. Too little space, too many roosters, can lead to ugly and even dangerous behavior.
Mrs K