You have to break your questions into managable ones if you expect a reply -- you've asked about 100 different things!
Also more than half of your questions are answered in the reference you were provided (you do have to read carefully).
Finally, you've given little info, for example the OW fares differ by country (the circle fares are not avaialable in all countries and have different terms).
In general the OW fares are sector based (not mileage), but there is one OW mileage based fare (the cheapest one). By contrast most other airlines have mileage based fares (Star and Skyteam, for example).
The OW sectors can be upgraded, using whatever upgrade is offered by the carrier flying that sector. When class of service is not offered, the rule is to downgrade to the lower class of service for almost all airlines (not just OW), but there are some great exceptions, and AA upgrades to F which is a great benefit.
A sector is a single flight number -- doesn't matter if it is your choice or the airline's choice. This makes those silly "same flight number with a change of plane" routes highly desirable at times.
The continents are defined in the rules, and vary by fare type. For the OWE Turkey is part of Europe but other middle east destinations are limited to 2 sectors.
SW Pacific is defined as Australia/NZ and Polynesia; Indonesia is part of Asia, for example.
Codeshares are allowed if both selling and operating airline are Oneworld (QF/LA qualifies, for example).