I'm glad that you asked:
1) Look, I will be completely honest with you. I honestly think, that Seance is a good card, because it's flexible. My real issue is that it's a spell and it has anti-synergies with Shadow Visions. You see, imagine when you are in a tough situation and you desperately need a specific answer. Against aggro decks you might want to fetch a copy of Spirit Lash with Shadow Visions, against Big Priest you might be looking for an Entomb. You don't want to see Seance popping up in the discovery pool, because it will lower the chances of finding the desired card and this will affect the outcome of the game.
2) In the past I ran 2x Psychic Screams, in order to counter Big Priest better. But right now I figured I need a slightly cheaper board clear (against some decks like pirate kingsbane rogue casting a board wipe too late will end up losing you the game). Even Shamans are ruthless and you need to deal with their board ASAP.
3) Regarding Mass Hysteria - I was also skeptical about this spell for the same reasons. I also didn't like the RNG aspect, but it proved to be more useful as expected. In some situations Excavated Evil is better (against some tokens decks), but Mass Hysteria allows you to deal with all kinds of boards. I remember the first time I used it I was facing a tempo mage (that had Kirin Tor Mage, Flamewaker and something else). If I cast Excavated Evil, I would have left the Flamewaker alive and it would have killed me. Thx to Mass Hysteria I managed to clear the whole board, to survive and to win thx to Amara, Warden of Hope. Mass Hysteria is extremely useful against big priest, even shamans and sticky boards in general.