Mirroring Objects - Strategies & Issues

Hopefully someone can give me some suggestions on the best way to approach this problem:

What is the best way to mirror a group of pro builder meshes? Here's the scenario:
I build a stairwell on one side of my environment and I want to duplicate it and mirror it for the other side. The issues I'm running into is that if I select all the objects and use the pro builder mirror tool, it mirrors them based on their local transform and not the selection transform so they get all jumbled. Is there a way to mirror them based off the selection transform? (maybe I just missed it)

I've also tried grouping them in an empty game object, duplicating that, and just changing the x scale to -1. This works, but when I try to freeze all the new objects transform so they won't all have the -1 scale on them, faces randomly get flipped. Is this a known issue? Why does freezing transform do this?

I guess I could just not be picky about freezing their transform to get rid of that -1 scale. Did I completely miss how to mirror in pro builder based on a selection pivot and not all their local ones?