This is a pretty standard problem with Nanite and RT shadows-The nanite proxy mesh looks different than the full-detail mesh, so it has self-shadowing artifacts. No real way to fix it other than crank up the proxy mesh count, but that erases most of Nanite’s gains anyways unfortunately.
You can set different materials and meshes to recieve RT shadows though, so you could disable it on the nanite meshes but maintain it for translucency if you’d like.
As for reflections? Depends on what engine version. Translucent reflections just got enabled in github, but I only know that for reflections on translucency-not whether or not they’re visible in other reflections.