Don't count your chickens before they hatch applies here. You need to figure out why the others died (most have some that do) and you need to get your survivor to being out of the shell.
Personally I would add more eggs to the current incubator, but only after double checking temperature in...
Since he went to a flock with other roosters it may take many months for him to fully integrate with the other flock.
In my mind there are at least two reasons to leave him where he is.
First and foremost you gave him away. He is no longer yours and you have passed the decisions of how to...
The rust color alone does not mean male, in fact having it on the chest in some breeds is a indicator of female. A comb that size at 7 weeks... Not sure if any leghorn pullets would have that, but the red color and size of the waddles should be enough to prove males. Not sure how Hoovers...
If you are able to warm one spot to the needed temperature, keep them dry and out of the wind it doesn't matter what breed the birds are. My hens have raised silkies during the winter.
I do not have personal experience with Marek's, but the most successful Marek's flocks I have read about are those that breed their own most resistant birds. Most of those that I'm aware of do not use the vaccine (it is not practical in a backyard flock and birds may still virus shed).
The natural process would be to lay a clutch of eggs then brood, so to keep brooding with out laying does mean the hormones are not balanced right. On the other hand, hens that lay and lay and never go broody do not have the hormone balances that would be needed in nature either.
...they hang out at the pond during daylight hours, but other times they spend quite a bit of time foraging on land. Most spend the night in a 50'*300' open top pen with a couple of sheltered areas that they rarely use. Two ducks currently choose to be spend nights with my Silkies.
We have...
You can put them under a hen, but it needs to be a broody hen preferably around 3 weeks into setting. This may or may not work well. Trying to put them with a hen that already has chicks doesn't usually end well for the chicks. They will be small intruders that can't keep up.
I have no idea why she was on the move in the dark, but any chance of a hidden outdoor nest? And broody? That would explain not being with the flock last night. If so that grounding might break the broodiness.
The new picture posted makes it clearer that they are likely all the same breed. Red sex link is a good assumption. That fact you were told all girls also points to that
The younger the chicks the less likely the older birds are to pay attention to them. Be aware that by removing the hen from the flock and in warning to stay away from her chicks she may end up fighting some.
Since your chicks are already 4 weeks old make sure there are places for the chicks...
I wouldn't worry about one being a Cornish Cross. The AI just gave two examples of yellow chicks that would look completely different from each other.
If the larger chick is the same age as the other two it may be a meat breed, but some meat breeds (I would not recommend to keep a Cornish X)...