Year of photos lost around transition to Photos app from iPhoto

Almost a year of photos have disappeared from Photos. Oddly enough, I'm also missing Time Machine backups for roughly the same time period, so no luck of recovering my photos there either.

I discovered this already in late 2016, but I did not have time to put myself in it then and being sure photos can't just disappear, I decided I'd hunt for them later.

Things went weird with the OSX-update that introduced Photos instead of iPhoto I was using earlier, but I think Photos managed to import all photos into the new app. The old iPhotos app and library remained side by side, which to me seemed odd. A little later I discovered that also a second Photos -Library2 -folder had popped up.

I've gone through all Time Machine backups existing from around this time, and no library there has any of the photos I'm missing when looking under the Masters-folder in the library. I don't understand how both my photos can have disappeared AND Time Machine backups have gone missing for more or less the exact same time period.

This is what I've got:

• I've got all photos until 2.5.2015 in iphoto
• I've got all photos until 2.5.2015 in Photos Library and Photos Library2 combined
• I'm missing all photos from 2.5.2015 to 10.3.2016
• I've got all photos from 10.3.2016 to 25.6.2016 split up between Photos Library and Photos Library2
• I've got all photos from 25.6.2016 stored elsewhere

This is how it's split:

iPHOTO LIBRARY

PHOTOS-Library

PHOTOS -Library2

TIME MACHINE

• Last regular backup: 10.2.2015 (only iPhoto-Library exists)
• Odd single backup: 30.6.2015 (Photos-LIbrary shows up along iPhotos-LIbrary)
• Resuming regular backups: 15.2.2016
• Photos- library2 shows up 1st time: 20.6.2016

RELEVANT SEEMING UPDATES AROUND THIS TIME:

• 28.3.2015 iPhoto 9.6.1
• 3.4.2015 OSX Update 10.10.2
• 27.4.2015 OSX Yosemite "Return/Recovery/Rollback" Update (don't know the right translation, Finnish OS)
• 5.7.2015 OSX Update 10.10.4
• 22.8.2015 OSX Update 10.10.5
• 3.5. OSX Update
• 4.5. OSX Update
• 18.5.2016 El Capitan 10.11.5
• 13.8.2016 El Capitan update 10.11.6 (still on this OS currently)