Good point. The rockets are rolled horizontally and then lifted vertical in many cases. So the payload and the adapter should be designed to handle that.
Yep, and basically every satellite goes horizontal at some point during their ground phase (integration and test). Big satellites actually spend most of their lives horizontal, mostly because access is easier.
How do you know these level of details?
I wouldn't be good at my job if I didn't.
(I've been, under various hats, conceptualizing/designing/building/testing/managing/launching sats--mostly big ones, though more recently mega constellations--for 20+ years)