I would use the ecoglow as a backup just as you said you wanted to use it. Put it in the area, but not necessarily right next to her, just close by. You should be able to tell within a day or two after she leaves the nest if she's able to handle that many chicks. I think the bigger issue with so many chicks is not keeping them all warm (it is summer after all) but being able to keep track of them and protect them from other birds and predators (if you have any). You've raised a lot of chicks this year, so you know how active they get and so many will make it so that momma will inevitably lose one or two in the shuffle. I've noticed in the past that some say that momma will leave the larger group to protect one and then the larger group gets in trouble. But, if you have everything controlled (i.e. a separate space for her with her monsters) they will probably be fine. I know being in Washington that you have lousy weather most of the time, and I'm sure you have plans for the chicks and mommas when the weather is bad. So, that is what I would do but keeping in mind that the ecoglow might cause some of the chicks to just give up their space under her since they have another source of warmth and may need to be brooded by you if necessary. But let us know how it goes, it should be interesting.
I have brooded Brahmas under a really nasty mix momma. Brahmas are very docile and the nasty mix was just plain nasty, evil and a bird that I would NEVER want any babies from. They Brahmas stayed sweet, but as soon as she indicated that she was through brooding them, I took them away from her and handled them for a few days before letting them go back to join the rest of the flock. As large as they were at 6 weeks they didn't have any trouble assimilating back into the flock. Since you free range, I would guess that you could do the same thing and the RDs will not have any problems getting back into the swing of things.
I only know what I've read here, no personal experience with grafting chicks so early in the brood. I would base my decision on how many of the day 8 eggs actually hatch. If a bunch of them do, then I would put them under her a week early and see what happens. Since right now you only have 5 eggs to put in the bator this week, those 5 should they all hatch (and guessing from your luck with the Langshans you may only have 3 hatch) you would only have a few to take care of in the brooder rather than a bigger batch. Another though, have you considered putting one or two of the eggs under her to allow her to hatch them out? Seems to me that she would be more likely to accept chicks that hatched under her rather than to try to graft them. Then, if she accepts the two that hatch under her, you can graft the rest that hatch in the incubator onto her knowing that she is accepting chicks. After all, chickens can't count, can they?
