Q: If I migrate to Photos.app (using OS X 10.10.4 and iPhoto 9.6.1), may I delete iPhoto and its library - will that delete all my photos?

Do not delete the library or you will lose all of your photos unless you import the entire library into Photos.

I would still keep the library as a backup, at least for a time until you feel comfortable removing it.

Yes after migrating  and nothing - but probably  not a good idea and it will not ave much space since the photos are not duplicated but connected via a hard link - Photos saves disk space by sharing images with your iPhoto or Aperture libraries - Apple Support

Deleting the iPhoto library has little advantage and if you want or need it back you can have difficulties

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