To begin with, here are the answers to your questions
1. SET NL 100 is correct
2. No, not required.
3. Using negative is fine.
4. I added the command SET ITERLIM 11 to the file which allowed the software to carry out one extra iteration over the default of 10 iterations and that resulted in all the cases being converged.
You may also consider defining the MEMBER TENSION after all your primary load cases ( after load case 14 ). A PERFORM ANALYSIS and CHANGE followed by MEMBER TENSION may be added.
A modified file is attached for your reference.
(Please visit the site to view this file)