The issue here looks to be that there isn't actually a page load associated with adding to the cart in v10. If you have the minicart on, the customer stays on the same page. If not, they go directly to the page that Google associates with starting checkout. There's no page view for adding something to the cart in the middle, and the funnel gets all thrown off.
Google Analytics has a feature called 'virtual pageviews', where you can send them 'fake' page views when customers perform certain actions that don't actually cause a page load. It was designed for Single Page Applications, but it's the same idea.
We've registered an issue in our internal tracking system to see about adding support for those virtual page views (or resolving this another way if that doesn't seem to pan out) in the future.