How did Orochimaru summon the 1st Hokage?
Orochimaru used 4 Zetsu bodies as bodies for the 4 hokages. Then, recall how each one of them were sealed in the death god? The 4th sealed himself with part of the kyuubi, and the 3rd sealed himself along with the first two when fighting against Orochimaru at chuunin selection exam. Thus Orochimaru can pull them all out of the death god and put them in the Zetsu bodies.
Why couldn't Hashirama break the seal?
Madara stated that you can free yourself if you know the seals to Edo Tensei. Hashirama probably knew the seal (as his brother created the jutsu), but he didn't even need to use it. Orochimaru specifically stated that he couldn't even control Hashirama to begin with. This is probably due to Hashirama being incredibly strong as well as Zetsu bodies being made with Hashirama cells.
Why is there so much mystery about Hashirama's power?
Because he is considered the god of all Shinobi. And since Naruto takes place in an era where he is dead, no one really knows his true strength. Of course, the only person who would know his true power is Uchiha Madara.