I think that if I were you I would use a Shared Library.
How to use iCloud Shared Photo Library on iPhone or iPad - Apple Support
One owner designates a part of her Library as the Shared Library, and the other can sign in with full access. The other part of her Library is the Personal Library, and it isn't available to others. I don't use a Shared Library, so my experience is limited to what others tell me, but it sounds like what would work for you. Each person signed in has complete access to the Shared Library, with the ability to add, delete, edit, add captions, etc. The Shared Library gives access to the owner's actual picture files rather than copies, so you have to trust the person you're sharing with.
Shared Albums uses lower resolution copies of your pictures, and much of the metadata is removed to protect privacy. When I've used Shared Albums, I have seen that they do not have Titles or Captions that I had added. This picture
is 5526 × 3108 in my Library, but in a Shared Album it's 2730 × 1535, which has 1/4 the number of pixels. It might be OK for an 8x10 if you don't look too closely, but not really a larger print. The original can make a 16x20 print or even larger--since you tend to start farther away from a big picture.
This picture does have the right date, however, so I'm surprised that you're seeing a date change. Are the odd ones loaded in a different way? Photos uses the date in the metadata, but Finder and other apps use the File date.. If the picture leaves the Photos system before going into the shared album, then the dates might change. This happens when pictures travel through texts and email, for instance. Could that be what's happening?
