Status Update June 20, 2015.
Well - I talked to Apple Support and Apple level 2 support. I have spent a couple of hours over the course of weeks uploading files to Apple with the promise of a solution. So after a few weeks, unfortunately there is no solution. Photos simply does NOT work as advertised and the migration of Aperture to Photos is simply not viable for large libraries of photos.
From my perspective, the problem has not been adequately engineered or beta tested. This is too bad.
Honestly, though; I think that Apple has chosen to move away from "pro" level photo management. The new Photos app looks like it is designed for photos from iPhones and iPads - and its is almost "hostile" to photos from other machines. This many be strategic from Apple's point-of-view.
I'm going to keep photos alive on my Mac for the innumerable quick snaps that I do with my iPhone. It is quite convenient to be able to have SOME of those snaps on all my devices all the time - but I think that I'm going to turn off the sync feature of Photos. I don't ALWAYS WANT ALL PHOTOS on all my devices all of the time.
For Snaps that I really need access to from other devices - Evernote and Dropbox actually work better and more reliably for syncing. This makes photos more of a mildly convenient app that won't see much real world use.
After a lot of reading, I've decided to pony up for the $10.00 per month cost of using Lightroom on all of my devices. Lightroom syncs photos, and I can choose how they are synced and where they go. Lightroom also allows you to do a lot more with your photos from organizing to modifying - and it actually works a lot better than aperture. Most high end software (ie. MacPhun Products) work very well with lightroom as well.
I have started the import of my 500+Gb library to Lightroom and it is working perfectly. Not even one glitch - though it does take a couple of days for the computer (and mine is FAST) to do the import using a thunderbolt-connected external drive (LaCie 2Tb tough drive).
So - at the end of the day, I have learned enough about photos to realize that it is really not ever going to be a viable replacement for Aperture. If you really want to quit wasting time and to get something that is likely to stick around and to keep evolving so you never have to do another mass migration like this again - abandon the Apple photos software and sign up for Lightroom. Photos may get "working" again so it can import Aperture libraries, but at the end of the day - it will likely never be an adequate Aperture replacement. Lightroom is the way to go. Yes, it costs.....but the stability, quality, security and functionality of the platform are worth it.
Sorry there was no Aperture solution. I'm glad I made the effort, because the process really got me to thinking about this - and I'm glad I learned enough to make the migration to Lightroom!