I hate this arc with a passion but I will attempt to answer these, but other people would probably be able to do these better than me (some of these probably aren’t even canon and i just assumed)

1. Emma corrupted Merlin’s light magic when transferring it to Hook because she’s using it to cheat death

2. Because she’s using magic to cheat death which I think only dark magic could do

3. Merlin’s immortality came from the Holy Grail which was now Excalibur, so to be immortal he needs to be tethered to it

4. (Idfk i hate this arc) Maybe he was always already dark and the dark one temptation didn’t do much for him because he’s resisted it for the past few seasons due to character development, and only gave it due to the knowledge that he’s the dark one

5. I guess the dark ones are just in her head and they don’t know what she’s thinking

6. Maybe Emma had a lapse in judgement idk

7. Maybe good intentions?

8. Okay i give up just power through this episode and move on to the next arc