Are we able to append fields to existing forms using the update endpoint
You add with the /fields.json endpoint.
How is existing form logic handled when updating a form? i.e. layers of visibility rules and progressive profiling
There's an endpoint for VRs, /visibility.json. ProgPro is considered a pseudo-field in itself.
You can learn a lot more by reverse-engineering forms. The methods are not all totally clear from the docs (though, to be fair it, it's one of the most complete doc sections on the Developer site -- it's just really complex).
How can we mass export all approved form names with their ID numbers?
The main /forms.json endpoint.
Is it possible to update all of the Marketo landing pages where these forms live at the same time?
Approving LP Templates and LPs is done via those other endpoints, not the Forms endpoints.
Is it possible to see what non-Marketo pages are using our forms?
No, not if you haven't already set up some kind of auditing process (i.e. using the Munchkin API to send an activity to Marketo when a form is loaded).
Thank you, Sanford! We'll take a look through all of this.