Can I Upload My Photos Back to Google Photos and Keep the Original Dates?
Only if the dates are inside the photos. Google Photos ignores the .json files it gave you. Here is how to make the upload work.
Yes, but only if the dates are inside the image files. Google Photos will not read the .json sidecars, even though Google wrote them.
On upload, Google Photos looks at the EXIF block inside each photo. If it finds a capture date there, it uses it and the photo lands in the right year. If it finds nothing, it uses the upload time, and the photo lands today. The sidecars sitting in the same folder are ignored entirely. This catches everyone who exports from one account and uploads to another, and it is why so many people end up with a second library that is completely out of order.
So the fix comes before the upload, not after. Run the app over your Takeout folder and it writes each photo's real capture date and GPS into the file itself. Then upload. Google reads the EXIF you just gave it and rebuilds your timeline correctly, with locations on the map. Fixing it afterwards means editing dates by hand, one photo at a time, so this is worth getting right the first time. The first 500 photos are free.