Okay, I am going to attempt to answer some of your questions based on what I have seen with what I am raising or what I remember reading....
1. It can take a good 7+ months for the chickens to finish developing, so how long to keep them would depend on the amount of space you have, now with this being said you can always be culling as you go as you see things you don't like in them as they get older and bigger.
2. I am not sure about the mossiness in the pullets as I didn't see this with the ones I had. Hopefully someone with better understanding will have some more ideas for you.
3. From what I have read, some of the white feathers may molt out but it depends on how old they are and if they still have to molt to adult feathers. If you have others without white or look better to you for what you are working towards than you could sell off as laying hens and make a bit of money towards feed costs.
4. not sure on this one
5. this I think varies greatly from bird to bird and some birds might stay solid color but could be hiding copper gene and might not show until future generations. The copper also can vary when it starts showing too- I have one hen that show only a very light copper coloring and she is almost 1 year old. My 1 blue cockeral is around six months old and he started showing copper around 3-4 months.
6. It seems like there is different opinions on the middle toe feathering- some recommend culling for it and others talk about the chicks lossing the middle toe feathering as they molt and get older. Again probably would take time, space to grow them out and then test mating to see if the next generation shows middle toe feathering.
7. If you are talking about the one at the back of the picture- I would guess it is a cockeral and is showing gold leakage- kind of a neat looking guy
8. the Marans are a very new breed as others have mentioned and it order to get good end results it will take lots of culling to get there. I think that with the more chicks hatched out at one time, the harder it is to give alot of individual attention to and they do get alittle flighty just like any of breeds can. From what I have observed with the groups I have hatched and raised is each group is different and the chicks within that group all have their own personalities- some are more friendly and others are shy.
Hope this helps and good luck with your chicks!
Yes, that helps a lot, just what I need. As far as them being flighty, yes definitely not enough attention, and I am getting spoiled by my Russian Spangled Orloffs--the growing birds get as little attention as the Marans, but they would mob me when I'd go in the pen to feed, I could hardly walk, they are so bold and curious, whereas the Marans would scatter.