Please see comments below to your questions.
1.) On the AdWords API perspective, what we currently have is the
guide for the integration of AdWords and Google Analytics. Please follow the guide on how to link AdWords and Google Analytics. In regards to linking AdWords to another APIs, I am afraid that I could not comment on this as this is already out of scope for our team. You can also direct your question to the
AdWords Community Forum to confirm what are the other APIs that AdWords can be linked to.
2.) AdWords API supports service account authentication as you can see on this
guide. As for any other Google APIs, I also could not comment on this as this is already out of scope for our team.
3.) This concern is already more specific to the Google Analytics API.
Since our team provides support specifically for AdWords API related concerns only, I suggest to get in touch with the Analytics API team via this link.
Thanks,
Vincent
AdWords API Team