Can I trust that if it says iPhoto library has already been migrated to photo library that it is safe to delete the iPhoto library without losing anything

I would at least do a sanity check and glance through clicking on a few images to make sure that they can load full size.

If they are precious images, it might do to get an external drive and save a copy as a just -in-case.

Note that you won't get as much space back as you thing since Photos uses "hard links" to refer to the originals, effectively sharing the space between the libraries.