I was thinking about that, but the main problem is that we do not store all the data which comes in.
If we ingest data from an app, I am pretty sure that the quality of the data is actually usable. But for example if we import an Records.json from Google Takeout. The quality of the earlier years is somewhat sketchy. For this we filter out some points like travelling with over 2000 km/h, sudden direction changes etc and they are lost forever. At least for Reitti they are unknown.
The feature would need a lot of explanation why the data we export is not the same we import.That is the reason I did not implemented it even if it would come in handy for testing stuff. Handling GPX files is a pita ...
Good question, afaik you can not enter multiple countries to Photon. I was hoping it would be possible but everything i saw was it is either one country or the whole world. But maybe you can have a look here: https://github.com/komoot/photon That is the service we are using.