I'll have to stare at this further because I don't use the Combo Viewer often (and I should!). Initially it wasn't showing what I wanted so I (mostly) left off using it.
That seems to be the major difference between why you see changes occurring where I don't.
The two parts of your post that I don't currently follow are:
1) drag a frame from the level strip into the xsheet switches the viewport content...
In my case nothing changes but that's probably because I'm not using the Combo Viewer.
I do think that undocked xsheets are important here because docked xsheets do produce changes whereas undocked xsheets do not (I'll have to check that out as time allows too)
2) If an option were available to keep the scene view displayed while dragging drawings from the level strip into the xsheet;;.
Here again, I'm successfully dragging and dropping drawings from the level strip into the xsheet and nothing is changing in the viewer (so I'm obviously not seeing the same thing you are).
Having said all this, I'm confident you know what you are seeing so the developers can address the issue.
In the first case it appears to be related to Combo Viewer while in the second (as possibly as in the first) it relates more to the xsheet).
My apologies if my attempt to look into the issue causes any confusion.
Yout workings are at a much higher level than mine and I'm still just scratching at the surface.