1) How is the SSPX any different from protestant and 'orthodox' churches who declared themselves to be the true Church?
Because the SSPX doesn't declare themselves to be the
true Church, they just claim adherence to the faith of their fathers. If their fathers (i.e. pre-Vatican II Church) was Catholic, the SSPX are Catholic.
2) Is the SSPX a valid and licit Mass?
Valid - yes
Licit - that's a complicated question. Ostensibly, no. But they claim extraordinary jurisdiction because of the present crisis in the Church. The entirety of the SSPX case rests on the jurisdiction question. If they have no jurisdiction, not only are their absolutions invalid but every exercise of the Sacraments, every Mass, is a sacrilege and a grave sin.
3) Is the SSPX sedevacantist?
No, most sedevacantist groups have split off of the SSPX, others were formed in a different lineage (Thuc). Some statements of Abp. Lefebvre seem ambiguous, however.
4) What is SSPX view of FSSP and other such extraordinary form orders
Traitors, turncoats, fake Traditionalists, "Tridentinists" (they only care about having a "pretty" Mass), Conciliarists, etc., etc. There is still much bad blood more than 20 years after the split between the FSSP and SSPX.
5) Who is in charge (temporally and spiritually) of the SSPX?
Jesus Christ, Pope Benedict XVI, Bp. Fellay
6) What is SSPX view on Vatican II?
It contained positive error (not just a bad "interpretation" or an evil "Spirit of Vatican II")
NB: I am not SSPX, I attend an FSSP parish.