I get that everyone in there is SS but what is someone like Merritt? Is Head TD not SS?

To answer just these parts real quick, I'm both. Senior Staff and the TD team aren't intrinsically linked, there's certainly overlap currently between the members of the TD team and SS but that's not really by design. People who are on the TD team may be nominated for SS partially on the basis of their work as a TD, but that's not any different from how a TL might be nominated for SS due to their work running their tier(s) or a PS staff member might join SS due to their experience handling the sim.
There are avenues for Senior Staff to communicate with the TD team outside of just having people on both teams, so it's not a necessity to have a TD who is also SS, but it's pretty common for at least one person to be on both teams.
Head TD does not come with a place on SS, it does come with a Community Leader position much like any subforum leads.
Hope that clarified this bit at least!