Eggs Due to Hatch in 4 Days, Slip Them Under Broody Hen?

Question:
Are 4-5 days of setting long enough for a determined broody to transition to mom mode?

Backstory:
I bought 15 eggs a few weeks ago to slip under a young broody (9 month old Welsummer, her first time being broody), and 15 ended up being too many for her. She's a small hen, and she evicted two eggs and ate two more in the first week. I settled on letting her have 8 and I put 5 in the incubator. So far there haven't been any more incidents, and the eggs are due to hatch in 3-4 days.

My reliable Buff Orpington broody started acting a little broody last week, and today decided to go full broody. She was a VERY protective momma hen last year, and already will attack my hand if it goes near her nest. I just isolated her and gave her the eggs from the incubator, and now the question is do I divvy up the eggs equally, give them all to the BO and break the wellie, or give them all back to the wellie?

The BO has never accepted feed-store chicks, and my main concern is that 3-4 days of setting won't be enough for her. She was broody for about two months straight last year (I finally caved and gave her eggs after I couldn't break her).

If the BO accepts the chicks, they will be 100% safe around the entire flock and be integrated quickly (she's at the top of the pecking order). The Welsummer has never raised chicks before, and she is near the bottom of the pecking order, although she has been setting consistently for about 3-4 weeks now.

The answer to your question is yes. I have given a broody chicks when she has been setting for less than a week.

As far as the wellie being a good mom, the thing that concerns me is that you say she ate her eggs. I would hate for you to give her eggs so late in the game and she crack them or otherwise discard them. I would wait until some of the chicks hatch and try to give her one or two and see how she behaves. If she accepts them then you should be okay.

I have never had an issue with my broody mamas being able to defend their babies, regardless of where they are on the pecking order. My little japanese bantam, my #1 broody, will attack even the guineas and turkeys if they get too close.

I would just make sure the Wellie and BO have separate areas to keep their babies unless you think they can get along. I have had hens co-brood, either together as partners, or just side by side and tolerating each other, with no problems. I have heard on here that other people have had hens kill each others chicks, though.