The mineup mod for 1.2.5 includes upside-down versions of the usual aboveground dimensions it adds:

In these upside-down dimensions, everything is vertically flipped (block models, fluid physics, entity physics, basically everything but the player) and the day-night cycle is also inverted.

Having these be added as potential immersive portals dimensions (as well as the three being there by default) would be fun since there's currently nothing but void below the nether.

Might be a possibility with Gravity Changer, now that it exists.

How can I have something like an inverted nether alongside a normal nether?

It's already a thing. You can make a dimension flipped in dimension stack and enable gravity change. https://qouteall.fun/immptl/wiki/Dimension-Stack

No, because dimension stack cannot clone a dimension. However you can use datapacks to add another nether dimension

...apparently that won't happen then

...apparently that won't happen then

This mod does not have inverse gravity feature. However the portals can have rotating transformation.

Could be done if this is: Virtuoel/Pehkui#20