It actually went totally fine and was a relatively painless process.
Overall I think they came out pretty good. There's two concerns I want to address overall about the rocks though. One is that because I didn't end up using the original assets I modeled for the rocks, there was no need to be constrained to their shape. Making rocks from scratch in Zbrush is easy and simple and I think I could have gotten way more out of the shape if I had gone down that route from the beginning. But because I sculpted these rocks with the original intentions of baking them down onto the low poly, I never explored the shape as much as I could have. If I have to do rocks again, my pipeline would go more along the lines of:
Model a high poly in Zbrush > Decimate a low Poly version > completely retopologise that version in 3DS Max or 3D Coat > Unwrap that in 3DS Max > Bake it down.
I think knowing this process alone may have been worth all the trouble it's caused me, I've worked out a zbrush pipeline that works for me.
The second thing I would address is putting more colour variation in the texture. The reference image I was using was just tone greys, but regardless I think I should have taken some colour liberties like with the mushroom and added hints of browns and reds to make them more interesting.
I also worked on water, which was an interesting experience. I had to make 5 new bump maps to create the different textures, then base colours that change at different depths to create the refraction on shallow areas and darkness at deep areas. I thought the way it reflected light was really successful- in my unreal scene at least. Once again the problem of working on different unreal scenes is while my water looked great in mine, when I recreated the effect in the official scene, it just completely failed. It was dark and ugly. A problem that I could have addressed in the process if I had been able to work directly with the scene lighting. But all the tampering in the scene couldn't get my water to react properly to the conditions being used. It's out of my hands now, which is a shame, because I'd love to see it working.