Help with iCloud Photo Library uploading

[johntimber]

Hello,

I'm having an issue that I'm completely stumped on and not sure how to proceed.
My mother-in-law ordered a new iPhone this last week so I thought it'd be a great opportunity to get her setup on iCloud photo library so we wouldn't have to do the usual backup/restore nonsense and could instead just sign in to the new phone and go. I was able to get all of the photos (about 7,000) uploaded from her old phone into iCloud no problem.
However, when I went to do the same to her Mac I ran into a somewhat frustrating issue. Her Photos library goes back to 2006 and has been migrated through multiple iterations of OS X and iPhoto. Her current setup is a late 2010 13" MBA with the latest Sierra. Her Photos.app library is 62.45GB and contains 18,000 photos/videos. We started the upload by turning on iCloud Library one night and the next morning it was stuck. We thought it might just be busy with a large video or the internet being slow so we waiting a few days but it still didn't move. I then brought the machine over to my house and attempted to kick-start it and ran into the same issue. The upload works until it gets to a certain state and then just stops. The progress bar will usually turn gray but sometimes it stays blue too.
I've tried turning the iCloud Photos Library option off and then on again. This restarted the upload process but again after a full night of uploading it gets stuck at the same position.
At this point I'm not sure what to do? Her longest video is only about 1:30 so it's not some super huge file in there. These are 99% iPhone snaps from over the years.
I've had a couple ideas of things I could try but I'm not sure how iCloud will handle it, so, some questions for the experts:
1. Is there anyway that I can find out what it's doing and why it's stuck? Like some sort of log or report?
2. Her new phone is 128GB so it has plenty of space. I've thought about using iTunes to cable-sync her entire Photos library over to her phone and then having the phone upload them to iCloud. Would there be any issue with this? What about all the duplicates that the Mac has already uploaded, will iCloud be smart enough to just keep 1 copy or will she end up getting duplicates downloaded back down to her Mac?
3. A similar idea, if the phone idea is dumb, what about exporting all of the photos to an external HD (or even just moving the Photos library file), opening it on a new user-account on my mac and then signing in to iCloud from that user account and trying to upload it from a different machine? I assume that if I do the "export unmodified original" option in Photos.app that it would leave all the metadata intact so there'd be no downside to exporting them and importing them into a new library correct?

Please help

I didn't find the right solution from the internet.

References:
https://arstechnica.com/civis/viewtopic.php?t=1336589
Teaser video company

Thank you