I am confused by this solution, instead of letting the mother raise her own chicks when she is more than fine to, you take them just before they're due to hatch?
The better option would've been to let her continue sitting until you're sure that they've been there about 4 weeks, like you were previously told - then you would know if they're truly viable or not.
Obviously it's your decision but I feel you've made it more complicated than it needed to be, harder for yourself and that poor hen probably thinks she was doing it wrong when she may have been doing just fine, and misses out on the opportunity to raise her chicks she's sat on for almost a month.