Is there a scripture to indicate a female's circumcision is invalid?
No.
And yet only one was circumcised by a woman.
Because he struck the rock more than once, disrespecting God. The uncircumcision episode was fixed by his wife's circumcising.
God commanded circumcision of all males to Abraham and his seed. If mothers are Abraham's seed, the command was to them also.
Does the text say Zippy cut off her sons' foreskins, or her son's foreskin? I think she only snipped one, but I agree, she might have done the other on a previous occasion. Perhaps she was trying to set a record as the first mohelet, and was running a business on the side, doing entire tribes? Or maybe not! Scripture doesn't say.
Exodus 4:24 At a lodging place on the way, the Lord met Moses and was about to kill him. But Zipporah took a flint knife, cut off her son’s foreskin and touched Moses’ feet with it. “Surely you are a bridegroom of blood to me,” she said. So the Lord let him alone. (At that time she said “bridegroom of blood,” referring to circumcision.)
If Moses' sons were not allowed to perform temple service 'cause one was circumcised by a female, why were both sons prohibited?
There is no scripture that says circumcision from the mother is wrong.
So presumably, the Jews would have expected that from his mom on the eighth day, right, given his dad was a Greek? Isn't the eighth day when circumcision is supposed to be done?
You're telling the story, and apparently not. But if both sons were not accepted, how can it be only one son was circumcised by a woman? If it was a mohelet issue, presumably one son would have been allowed? Or you believe Zippy circumcised the other boy at an earlier occasion?