Haha wow I give up. I spent about 5 hours trying to copy this map, mirror it and attach it back to the other half.
It would crash when I tried to mirror the other half within the same map file, so I mirrored a duped map. But every time I try to copy from one map to another it will crash. Sometimes the mere act of copying it crashes Hammer. When I DO get it to paste in, it refuses to save properly. I'll ram CTRL+S but when I close the map and reopen it, the flipped portion is gone. Trying to copy/paste in smaller portions just gets me the same result.
I feel like I'd still be far behind everyone even without this, just looking at some of these makes me want to shrivel and cry. Just for the sake of showing I tried:
Anyway, good luck to everyone else!