hrmm as to the whole cursed seal thing i always saw it as a transfer point it transfers snakemans power to sauske but it damages sauske at the same time kinda thing and since orichimaru is in a battle for his life when sauskes will broke wouldent that mean that orichimaru could drain sauskes power
and for the frog thing i believe that it wasent that he couldent he could easily summon him but not controll him