Because every model is different? It’s like saying “my car can go 100mph, but slowing down going around a corner makes no sense to me.” The printer in theory can still print at the stock speed, it’s just going to make noise because of the reasons I showed you. It’s too many short movements in the arc of the circle. If you want less noise, you have to slow down that particular part of the model (more than likely the infill).
The black part is threaded. There is no thread behind that. Nothing to mount a screw into.
Looks to me like these are for attaching P1P side panels. Because the P1P side panels are multiple pieces, they have threaded spots in multiple places for multiple attachment points. The X1Cs are one huge side panel, with no additional threaded mounting spots.
So again, I don’t think your printer is missing any screws. And I’m pretty sure that your noise issues are what I described above.