Something I do know is that one normally sees a sharp, accurate contrast when one checks the differences between mundane aspects (what's gained, what's lost) in different locations. At the very least, this means that there is a difference in strength (even if old ones don't disappear entirely, they at least weaken, it seems).
The main question is whether natal mundane aspects persist even if weakened. I'm less sure about this. My bias is that they do persist, but this might be just seeing the residuals of established character patterns, i.e., once someone has spent years acting and responding in certain ways, these tend to last (at least for a long time) in the personality independent of whether an aspect persists. While I know that new angular planets are more or less instantaneous (people can feel natal planet angularities merely driving through an area), mundane aspects may need a time to stabilize through actual acted out behaviors.
For the entire 900+ study charts catalogue, I have every Class 1 natal aspect catalogued (of the 10 core planets), including Class 1 hard aspects where there is not a matching Class 1 ecliptical aspect. This allows preparing lists. I'll post something in the next comment that might be useful in drawing a conclusion.
I have no basis for concluding that midpoints (other than those involving angles) "exist" in a mundoscope. They may, or may not. But, yes, I've seen that these tend to come to the foreground together in an SLR. - Sedna is tightly connected to Ju-Ur-Ne anyway (ecliptically), and the Mars-Sedna is much stronger mundanely.