Single User mode (10.13.6) write locked with SIP disabled
I can say that the problem was indeed with SIP. 


  status: disabled (Apple Internal)
  should apparently have been status: disabled without (Apple Internal).


Unfortunately, I only got so far by retrying several times and mixing up the order of some steps up to step 7, and can not with certainty say what has fixed it.

Now moving on to Part 2.