what is the source of federal power to supersede a governor’s powers to regulate an emergency in his own state?
The Fourteenth Amendment. The equal protection clause in this amendment has been held by SCOTUS to incorporate all Constitutional rights at the state level. Even in a state of emergency, the rights granted in the Constitution cannot be infringed absent just cause. Certainly prohibiting drive in church services, for one example, is a blatant infringement of the right to free expression of religion. There’s NO safety issue involved since nobody is coming into personal contact.
A state of emergency gives governors extra powers, but, contrary what some seem to believe, it does not grant absolute, dictatorial powers.
Yes, it seems that someone must have standing to sue, so it would have to be citizens of any particular state doing the suing. Of course we all know there are groups that help individuals to do this.