I doubt rank matters much above Full Member when you're a trusted person/Staff member (Which is the case here.) Businesses who want to boost their reputation a bit are more than willing to pay such amounts, i'm sure.
Since you're only a Sr. Member, it's going to be tough to get $100/month.
Depends. Chipmixer's campaign/BitBlender both offer more than that. (These are just two that i was able to come up with out the top of my head, i'm sure there are more.)
Because it doesn't matter. Senior, Hero and Legendary signatures don't differ much and offer the same visibility in threads. Being a staff member is like a premium because people tend to read things posted by you and focus more on the signature and avatar than if you're a normal member.
Of course there are many campaigns that pay more than 100 to Sr members. Yolodice pays $200 per month (if we count Bitcoin at 5000 USD).
I'm not certain which rules you find difficult to follow; but to each their own.
Maybe he wasn't comfortable with this part:
I can remove you from the Campaign at any time, for whatever reason
I sure wouldn't be.