Yes. Sirius was the second member and was in control of the domain and hosting for an extended period of time. After that, Theymos took over as the administrator in 2012 IIRC.
Yea, that's correct as theymos has confirmed it
There was a period where both Sirius and I were active administrators. For convenience, I reckon Jan 1, 2012 as when I became head administrator, though it was actually a gradual transition starting before and ending after that.
Satoshi is indeed the founder, but he is not the first member on this forum. Here is him , user with UID=1
admin