I think the agreement between Schulz and Django regarding the Brittle brothers was for Django to earn his freedom in exchange for identifying them. After he shows skill as a gunfighter, Schulz takes him on as a partner/protege. The next bounty is Django's first as a full fledged bounty hunter.

I think I can help with a few of your questions. The order of the bounties isn't important. It's Django learning the ropes of being a bounty hunter to establish him as gunfighter so his big rampage at the end isn't out of nowhere.

Steven's position in Candieland is essentially he's a collaborator who enjoys some privileges due to his long standing relationship with Candie. He's far to old to labor, so he maintains his position through guile, flattery, and degrading others.

The money isn't the issue with D'Artagnan and Candie. It's about Candie having the power of life and death over D'Artagnan, brutally intimidating Schulz and Django, and establishing Candie as one of the movie's villains.

Schulz kills Candie out of contempt and probably some wounded ego after his plan to con Candie into releasing Hilde fails. There's also a good chance that Candie's men were going to kill them anyways, so Schulz may have just wanted to take Candie down with him. Tarantino deliberately avoids giving us clear answers in that scene; Schulz's "I couldn't resist" is meant to open to interpretation.

Heart of Ice still holds up too. They made Freeze into a tremendously sympathetic, complex villain.