Inconsistent egg production, extra thick egg whites, hens losing neck feathers...newish chicken owne

About a year and a half ago my husband and I bought a house and inherited two hens. They were both hand raised, very tame and friendly. Unfortunately we lost one about a month later to a wild predator. We decided to get a few more hens in her place. We got three new girls from a local farm right around laying age. They were definitely NOT hand raised and not people friendly. Now they will come when called and don't mind being around us but run if we get too close.

We used to leave their pen open and they wandered the property as they pleased but quickly learned that 4 chickens all day every day can make quite a mess of your lawn. Now we let them out on the property a few hours a week but they are never confined solely to their coop, as you can see from the pictures they have quite a large enclosure which they always have access to.

We feed them a layer mix from the feed store, pellets with shell. They also get "treats" as shown in the picture a few times a week, and kitchen scraps whenever we have them.

I did notice the other day that there was a mouse in the enclosure, and we have a big problem with the wild birds coming in to eat the chicken feed (which is why we switched from crumbles to pellets although they don't seem to mind).

So here are my questions...

1. The three new girls started out laying like pros but within a few weeks began to taper off and seemed to lay whenever they felt like it or not at all. They laid occasionally through the winter. Now that the days are getting longer and the weather better they started to pick up and were laying about an egg a day each, but once again have slowed down and now it appears that only one is laying regularly. The black silkie has only laid a handful of eggs the entire time we've had her but I don't know her age. Are we doing something wrong that is causing them to have such inconsistent egg production??

2. All three orphingtons lay different eggs- one small brown eggs, one medium green eggs, and one large pinkish eggs. The large pinkish ones occasionally have hard buildup on the outside of the shells, they look like tiny dots of shell material. Sometimes it's one or two, sometimes it's a whole pile of them. Her eggs also always have SUPER thick egg whites. When I break the egg it's almost a gel-like consistency. Is this something we should be concerned about? It wasn't that way when we first got her.

3. The two more red girls are starting to lose feathers on their necks. After looking it up on here I saw several people say it could be lice, but I can't get close enough to them to really see anything. The other two, the blonde and the black silkie don't appear to have any feather loss.

Please see attached pictures. And don't mind the poop, tomorrow is coop cleaning day. I rake the pen and clean out the coop and bedding once a week. I'm open to any suggestions for any of these issues or anything else you may notice in the pictures. Like I said we're new to the whole chicken thing and I'd love to learn from some experienced chicken folks! Bascially.....I have no idea what I'm doing. Thanks in advance!!!