1) I recently purchased RHA T20 earbuds. On the pricy side but well worth it IMO. I also use PowerAmp.
2) It would definitely be nice to have multi window functionality ... hopefully this is an area Google is focusing heavily on.
5) I bought mine from Google, but I've heard Huawei customer service is atrocious. It was well known that the 6p would be expensive and difficult to repair, so I can't sympathize with anybody that didn't purchase insurance.
Nexus Protect is priced fairly, and IMO it's well worth it for the piece of mind alone. And compared to Asurion it's actually cheap.
RHA make some great headphones, I have the MA750i & MA600i and can personally vouch for their incredible after sales care.
Huawei customer service is appalling (read does not exist in the UK) unfortunately there is no option to purchase Nexus Protect here, at least there wasn't the last time I checked. Thankfully (& this is the only time I'll say this) I got mine through a carrier so it's there problem if it goes wrong.