07/23/18 Update: Wanted to add a bit more to this answer for this who come across it in the future:
Updates (via the Editor or API) to Landing Page Content, Variables, & Meta Data will all ADD to an existing Draft. Sequential API calls for updating Content, Variables or Meta Data will all ADD to an existing draft.
Update and Approval of a Landing Page Template (via the Editor or the API) will spawn a new draft for each Landing Page that uses that template. This new draft WILL OVERRIDE any previous drafts for that Landing Page.
So, in my case of mass updates via the API to both templates and Landing Pages, I will need to Update and Approve all templates first, and then approve the LP Draft that action creates. Then, I can update my LP Content, Variables & Meta Data, and approve that draft.
For anyone who comes across this post with the same question, I got confirmation form Marketo Support:
"If you update via API, the previous drafts are effectively discarded and the new draft is put into place.
Hence if people have any current drafts, and you update the template, their work will be lost. "
"You will need to update your templates, approve that change, and then update the Landing Pages, and then approve that change. "
This means that if you are updating content across Templates, Page Content and Page Variables, you will have to Update and the Approve for each section.
You will need to update your templates, approve that change, and then update the Landing Pages, and then approve that change.