Check out
https://mymotorcycleadventure.com/ , a Norwegian I met a few months ago in South America. He will know the details of traveling as a Norwegian in different countries.
3) I only paid 1 very small bribe (related to not having proper insurance) , borders were often slow but the only requirement was patience.
4) us, Canada, Mexico I think you will need to buy in advance. Central America was either not required or purchased at the border, but probably worthless in case of an accident. Columbia was required, Ecuador doesn't need insurance anymore, Peru, chile, Argentina, Bolivia, Brazil, Paraguay and Uruguay - you can get a policy that covers them all. Finding an agent was tricky for us , we didn't get the policy until Bolivia. Supposedly you can buy it online but I never figured it out.
5) from Mexico to Argentina , the cost of living can be very low. Argentina and chile more expensive, and Canada-USA are crazy, but not as bad as Europe!
6) September is early spring so it shouldn't be an issue. Spend lots of time in Ecuador, Peru and Bolivia to start and you shouldn't have weather problems. The weather issues there are usually related to road closures from landslides caused by rain, so just pay attention to local conditions at the time of travel.
7) stahlratte was a highlight of our trip.