We painstakingly build websites to be fully responsive, mobile first, and very SEO friendly. They also "Gracefully Upgrade", i.e. the mobile site has everything you need but hides anything that isn't of primary importance, whereas larger devices display additional helpful and relevant content the larger you get.
For instance, on a large screen our event tiles will include descriptions, but on the laptop size, the descriptions are hidden from view to keep the layout tight and focused.
If a Googlebot uses a laptop sized viewport, then that description content would be hidden (css) from view which I assume would result in a reduced score, or simply ignored valuable content that should ultimately be used in our page rank calculations.
QUESTION 1: Am I incorrect in assuming that Google will ignore our responsive hidden content? Or will they possibly see it but reduce it's relative score due to it technically being hidden?
QUESTION 2: If the viewport size does come into play with what Googlebots will see, what viewport size do the bots actually use when loading our site?
Am I incorrect in assuming that Google will ignore our responsive hidden content? Or will they possibly see it but reduce it's relative score due to it technically being hidden?
No. Google will see any content your page serves up. CSS tells a user-agent how to display content. It doesn't physically hide that content from them. So if your server serves up that content, even with its CSS hiding it, Google can still see it.
If the viewport size does come into play with what Googlebots will see, what viewport size do the bots actually use when loading our site?
Googlebots does not use a viewport size when "viewing" a page. They are parsing the document for content, not rendering it.