Halvorson could definitely benefit from a good defense behind him given his GB tendencies, and the fact that he throws 100 mph avg heat.... that's kinda cool. Doubt he's available, but it is Colorado after all....
As for the odd man out if everyone is healthy... it would have to be someone with options... or Burr.
If you assume Green gets turfed (not a guarantee of course...) all of the RPs have options with the exception of Hoffman, Yimi, Burr and Yariel.
So your 8 guys in the pen would be Hoff, Yimi, Burr, Yariel, Little, Fluharty, Sandlin, Fischer right now. Schultz and Bruihl would be the odd men out.
If Burr comes up and stinks, PUNT, and bring up Schultz again. If Fluharty keeps struggling, Bruihl steps in and Fluharty goes down.
If Green isn't turfed... it would likely be Sandlin sent down. I think they're keeping Fischer up since he's shown so well.
This becomes even more complicated when Manoah comes back since Lauer would likely lose his spot if Scherzer's thumb is still functioning. Lauer is out of options so he'd have to stay in the pen and become the defacto long man, forcing someone else down.
Luckily, these things tend to work themselves out because... name one pitching group in MLB that's ever fully healthy?