my dilema of the day....please help a new hatcher out

I gave my Broody BO Sunny 3 fertile Welsummer eggs I bought on eggbid on May 14th around 6pm, so today is day 21.
She has since broken two of the eggs, I have candled the last egg & I'm pretty sure there is a chick in it. Currently Sunny is in my small coop in the same nest she started in 21 days ago. As of a few minutes ago there were no pips, no cheeps & no change in the egg or broody.
I'm off from work today but will be working tomorrow. I have located someone in my area that will sell me a day old chick today because I'm afraid having just one baby will be hard. However I'm also nervous about going through the trouble of buying a chick & then possibly not getting a live chick from Sunny's egg. Then I'll have just one chick again.
I can't buy multiple chicks because DH has already reminded me we are at our space limits in the coop.
So question number one
Would you buy another & chick & when/how would you introduce it to the broody?
When would you move the broody & chick(s) to the nursery?

Thank you so much!

 

anyone have an opinion?????