I really enjoyed using the Pindigo app on my iPhone when it was able to sync with Pinside's score database. According to the Pindigo folks, reestablishing this feature would require Pinside to adopt OAUTH authentication, which robin has said is on his to-do list:
https://pinside.com/pinball/forum/topic/pinscores-app-testers-needed/page/8#post-3490645

It would be super-awesome if this change could be made to allow Pindigo to once again sync with Pinside scores! I for one would be much more likely to enter scores on Pinside if I can easily do so through an app like Pindigo rather than going through the web interface.

Thanks for your consideration.

I didn't realize feature requests were pay to play, but I'd happily kick in $20 to get this.

No answers to the rest of your questions though, I'm not a Pinside web developer. If you've got constructive insight to share on the questions you raise though, by all means share it!

Bump, would still love to see Pinside and the Pindigo app play nicely together, as is I find I'm not tracking my high scores either place, and I'd love to be tracking them in both!