No they do not. It’s a ritual led by the High Priest under Amon’s approval to wipe Nain’s past memory where they basically subdue him with incense and then he wakes up with a fresh mind. The reason Amon does this is because Nain has lived for so long and has lost many loved ones during that time, which has caused him to at times become unstable and attempt to end his life
Yes he escapes with Gwen at some point. Gwen is a powerful mage outside of Trastasa and uses his magic to help Nain escape with Lu (Rue in the manwha) and Yi Wu. Nain learns a lot about his past self and the outside world. Unfortunately Amon does find him eventually and to make a long story short, they are both alive at the end with Nain.
1) It’s in the middle of the novel. If I remember correctly it was after Nain realizes he has some feelings for Gwen in the middle of the novel.
2) He wants to but Gwen and Nain escape before he is able to harm him
3) Yes! The priest tries to drink Nain’s blood because he thinks it will give him immortality but it is actually very toxic for a human to ingest

Ask me about spoilers and I’ll do my best to answer them (●'◡'●)ノ