What happens when I jiggle the staff

Unless there's been a recent revamp to the Sea that I'm unaware of, the Seaside Shack is not accessible until level 11. I would suggest, however, that you not do any of the Sea content until level 15-ish as the monsters are very high-level and can hit pretty hard.

Passing through the mysterious astral gash after the main quest is completed will allow you to enter the afterlife but you cannot return to your old life so make sure you have no Unfinished Business.

Though there is some interesting content in the afterlife, there are no real quests or advancement or anything, unless you count the whole new adventure you get to start out on as a level 1 by ascending.

Ascension allows the player to "reset" their character to try out any of the other classes, each with their own content, as well as voluntary tasks or restrictions available only to players that have ascended. You will get to keep all of your non-quest items (though they will be unavailable for a certain amount of time) and some (likely one) of your skills.