I added your code to my test child theme and everything works no errors and no white page. The search is in the main nav the header aside is there and I can put a widget in it and the widget appears where it should( lacking styling ). I'm not sure what to tell you as everything seems to be working properly for me with the code you posted.
I'm not getting the white screen ALL of the time though. Most of my pages work, but some give me the white screen. Not all.
So in your testing, you may have that issue, but just haven't found the white screen-sensitive link, etc. (and that link is totally random - its just another page - even sometimes it works, then I go back and get the white screen.
Thanks for all the help - totally appreciated.