[Zeke]

Is that a meter next to the panel? If so, why do you say you must install the new breaker hot?

More: if there is a meter there, this is a main panel. I don't know where I got off track and considered this mess to be an existing sub. It doesn't look to me like that now.

Here's one more item to be corrected although you say the 100a breakers will be removed. If not, the 100 amp breakers could be for 240 volt lighting, in which case the unidentified wires should be marked green.