data value change via web service API

Yes.

You can also infer that these data value changes are a result of that "Fill Out Facebook Lead Gen Form" activity because they A) happen at the same time stamp and B) are sorted underneath that form submission.

Yes, Facebook Lead Ads updates do come through the Web Service API.

If you double-click on the “Data Value Change” activity, you can see additional details like the source and sometimes the Modifying User. However, in the case of Facebook Lead Ads, the “Modifying User” field usually won’t show because these updates are handled by the Facebook Lead Capture Service via an API token, not a named Marketo user. So you’ll typically just see “Web Service API” listed as the source.

Yes, Facebook and most other Launchpoint integration updates/form submissions have a source = Web Service API.