Cichol's unleashment was a byproduct of releasing the seals. Releasing the seals of Beokros and Siglint unsealed Elchulus, which when his seal was broken passively broke Morrighan's seal. Ironically, Keaghan not only failed preventing Morrighan's awakening, but accelerated it. Cichol was born after Keaghan broke the barrier of humanity by eating Verafim's heart, which causes the summoning of the dark god. By this, Keaghan inherited all of Cichol's power and ability, yet retained his personality. Basically what this makes it sound like is that everything he's ever done has been under the intention of bringing back Tieve. With that in mind, I'm doubly pissed that they cut off the main storyline in Mabinogi after Cichol began to inherit the power of an Absolute God. Considering that was Nuadha's intention in Falias, it can be assumed that Cichol sought out the same power.
It seems likely that Verafim had in fact taken over Elchulus's body after his heart was consumed. (I don't know about you, but mostly dead bodies don't keep talking).
Also, the goal of the Silent Brotherhood was not to uphold the prophecy, keep in mind that they're the ones who know that it's not true, but rather to protect the Oracle. Considering the Oracle has fulfilled the prophecy, I really don't know how the Brotherhood will continue.
As for not getting to Erinn, well, we were there the whole time. Elchulus states that they (you, Keaghan, and Tieve) are in Erinn right now (while you guys are in Heide). This is supported by Legatus' saying that Lann, Fiona, and three other warriors fought the dragons long ago. It would be impossible for him to know this if it occured in a different world. But then you might say, Erinn is a paradise and people don't die. Wrong. People have died in the Mabinogi storyline. It's likely that there had yet to be Milletians in Erinn at this time, however there are the Tuathe De Annans (mortal humans living in Erinn), so it's likely that the majority of the NPC's, if not, all, are Tuathe De Annans. It's a little bit difficult to discern whether or not the playable characters are Milletians. It can be assumed that because they did not appear in Mabinogi, nor posess the capability of rebirthing, that they in fact are not Milletians. Just really, really good fighters.
About Verafim taking over Elchulus's body, the camera was centered around Verafim, Elchulus is only shown flying away. I think Verafim is just able to live really really long without a heart...that or he using some spirit mumbo jumbo to talk to Keaghan after death lol.