Just a late follow up, after lots of burned midnight oil (much of it could have been spared with better documentation and more reasonable error messages), I was able to nail the ERP Upgrade process down so smoothly that I am now upgrading two systems in parallel.
But that story about "most time of the upgrade the user are able to work in the system", only holds true for end users.
Step "Preprocessing", as harmless as it sounds, takes up almost half the processing time, and during all that the system is locked for changes.
So our developers are up in arms, and I end up having to manually unlock the systems here and there.
Its not the way its supposed to be done, but that "minimum downtime" in Preprocessing lasts over three days(!), even on our fastest servers.