It is worthwhile to quicly verify there is no check-mark in the "Locked" box in File-Get Info as you described. In my case, it was not there, however.
And like you, I found that the chown and chmod Terminal commands suggested by Apple at If Photos can’t copy or create a library - Apple Support also did not fix the "Permissions" problem for me, and many others. Here is the fix that, at least for me, did enable Photos to open my old iPhoto Library (copied from and external drive to my Pictures folder); paste the following one line into the Terminal:
sudo chmod -RN /Users/$USER/Pictures/iPhoto*
This assumes you the "iPhoto Library" folder you want Photos to use is in your Pictures folder.
P.S. I added feedback to the apple site telling them "You need to add this additional commands, which did solve the permssions problems for me".
Be careful NOT to do the chmod on /Users/$USER/Pictures, as all the ~/* folders are supposed to have ACL's (to prevent you from changing the names of home-dir folders Documents, Downloads, Pictures, etc.)
You should not have to change $USER to your actual username, either. At least in my tests on 10.10 and 10.11, the $USER is still automatically expanded to my actual username, and not root, even when I use sudo, such as:
sudo echo $USER