+1 to the possibility to select a face and lay it flat on it. That would address more than 75% of the situations where you would normaly have to go back to a 3d soft, import the stl, arange it (that could take time too) and re-export it.
Yes, please add this! There have been several times I've needed this feature but ended up rotating tiny increments again and again until the proper orientation was reached. PITA!
I end up using Cura to break apart and lay my parts flat, then export/import into STL. If s3d had "split into multiple models" and "lay flat" I could do it all in s3d.