iCloud Photo Library Problems

I helped a family member get a new iPhone and one of the main reasons she wanted it is so that her data syncs flawlessly between her phone and iPad. Photos is one of the main things she wants. iCloud Photo Library should be the perfect solution. The problem is that she is switching from an android phone. I first tried syncing all of her photos through iTunes using my Mac and that worked beautifully until I went to enable iCloud Photo Library and was told that all of those photos were going to be deleted. Why??? Why can't it upload those photos that were synced with iTunes??? I don't understand this limitation.

Anyway, I did some more research and decided to upload the photos via the iCloud website. All was going well, although VERY slowly, when I went to bed last night. It had uploaded about 180 of the 879 photos. I wake up this morning to my MacBook fan screaming as fast as it will go and "Safari Web Content" not responding in the Activity Monitor. Sure enough, the upload stopped ar around 320 photos. They don't seem to be in any order as far as what was uploaded. There are some dates that only have one photo and others that have many and if I just upload over again I will have duplicates of all of them. Is there any other way to do this? Should I just delete them and start over again and hope it doesn't lock up? This is becoming very frustrating.