You can try to use the ability on anyone/thing regardless of condition. Your mileage may vary. I will point you towards a couple sentences in the general setting blurb on the After (pg.6):
"Ritualists can oftentimes communicate with the recently dead. They commonly describe their existence after death as most throughout the ages have hoped it would be: a gentle and possibly eternal rest."
Once you activate a second ability your previous ability can no longer be used unless you again pay the activation cost for it. For instance:
I have 6 Mana (Max attainable through build and itemization).
I cast "Immaterial Sigil" for 2 Mana at the start of an encounter. Whenever in a ritual zone I have Mundane Ward.
Later in that same encounter I decide to cast "Etheric Sigil" in order to deal with a nasty enemy. Now when in a ritual zone I no longer have Mundane Ward but may call "Piercing by Spirit" freely while attacking.
I only have 1 Mana remaining. Meaning that I do not have enough mana to cast any more Ritualist spells to replace "Etheric Sigil" until I next rest.
The next time I rest "Etheric Sigil" is dispelled, however, I regain all of my lost Mana and can begin again how I see fit for the next encounter.