Yes, that's not a problem. The scanner automatically assigns the screenshots to the correct day. So if you happen to have older screenshots of your trainer profile, you can add them as well. The more data, the better.
You get the best results if you take the screenshots at the end of your play day. Before closing the app, just quickly take a screenshot and scan it.
Trainerstats does not have access to the real game data and relies on the submission of screenshots to determine the data. Therefore, we recommend uploading the screenshots regularly to get the best possible result.
No, the levels 50-60 are not yet officially confirmed. It is an estimate based on the previous levels and should only serve as a guideline.