Response to an unknown guest (sorry for the wait)
I don't know anything about Sans Undertale or whatever media he's from, so I can't so for sure. As for jumping the shark, I think a good example of this would be Red vs. Blue Season 16. The idea of introducing literal Gods into a show that has modern day culture (Earth, pop culture) and time travel was really weird.
Response to another unknown guest
Yeah, I could see Rex and Jesse calling out Tema RWBY for their lies (especially Yang). I would imagine a shaky, tense alliance between them to stop Ironwood. They may not trust Team RWBY, but they know they want to save Mantle and the world. And Ironwood is pas the point of reason. I think if he shot one of the clones, the 501st would go to RWBY. I'm not sure if they're strong enough to stop Harriet, but they would execute her if they did.