I was able to reproduce a crash when making any change to root of a reparented Blueprint in 4.7.5 as well as our internal build, and I’ve created a bug report for it in our system (UE-13774). I’ll let you know as soon as I see an update on that. Thanks for reporting it!
other issue you’re posting about appears to be unrelated, so I’d like you to open a new post in Bug Reports section with same details and screenshot, please. I’ll start looking into that one in meantime. Thanks!
Just letting you know that this has been fixed internally and should no longer be an issue in 4.8, once it is released. Please let us know if you see this again after release. Thanks!