That's wonderful! She will settle down. Are the other chicks older or younger? In any case, she will be thrilled to have company once she gets used to them.
I would not use that incubator again without testing the humidity and the temperature. Even though the incubator "worked" to incubate, it did not do its job in the final stretch.
I do not think it was your fault. Temperature fluctuate and lower humidity can cause chicks to be week and die...
I don't know what it is. That is a bad sign. Brinsea incubators have a good reputation. They have scratch & dent sales on their website. A reliable incubator is an investment, but it saves little lives.
For the time being, you may want to just buy friends for your baby chick. Get pullets, not...
Welcome to BYC. I usually leave my eggs until day 25. I've had one that I thought was dead hatch on day 24. They don't have much room to move by that stage and you can't see well because the egg is full. If none hatch, maybe you can pick up a few pullets at a farm store to keep your little...