1.Immigration & Nationality Act
2.I'm not sure,I never meet the case as it -- apply
extention of H1B after 6 years in USA,but same time require CP of immigration overseas,but usually,USCIS approve the H1B extention case annually after 6th year,if your I-140 have been approved.
3.You should apply H1B extention to Nebraske,I think
no problem on it.
4.You still need your current employer to file the petition of H1b extention for you.