Yep it's ultimately up to them, however as a team we were invited to give ideas and preferences. I'd like to pitch my preference for Pop!_OS, so I'm looking for data to back it up. Anyway I'll follow your suggestion and ask more specific questions in the contact form, even though I don't formally represent my company.
Yep, Ubuntu was mentioned as an example in a few meetings and I think they will end up doing that. And it's fine, give me literally anything other than Windows and I will be happy, however I'm a spoiled kid, so I also don't really want Ubuntu.
I think most people only look at the gnome version when talking about updates. Then if their distribution ships a year old kernel version they don't even notice. System 76 is taking care of the user experience from the hardware to the software, a bit like Apple does, but in an ethical way, using free software and even open sourcing hardware. I've been running pop since I discovered it (in 2018 i think) and at this point I'm probably biased, but i think its quality is unrivaled. I really hope the release of cosmic will help you conquer all kinds of users, since, as we said, people only look at the desktop environment. About the rebase, ubuntu started stinking and some people won't consider ubuntu derivatives, it's just a perception thing. I see a rebase as a marketing move more than anything, but the return on the investment is probably not there.
As far as I remember, they said in some reddit post that the theming will be shared.
In our case we need Linux because the software we use only run there or is better integrated. Infact we use WSL for the core activities, bit WSL is crap.