Welfin also had a reason to switch sided, he hated the ants after realizing what they did to him and Gyro but his turn is a lot more grey since he is still looking for Gyro who we know isn't a good person and Welfin is aware of as well. I didn't care much for Hina but she sure didn't turn good, Brovuda is fine, his role isn't too big to see it as a flaw.

Heel Face turn means Antagonist joining the good sides and Face heel turn means joining the bad side. That being said Your second comment makes no sense whatsoever. In what way did Youpi switch sides? You do realize that switching sides means joining the protagonists to fight the King who Youpi loved, that doesn't make any sense? Youpi's stance more in the middle, he was confused about whether his power should be his or for the King but in the end he realized it was never meant for him. Also Youpi loving the king is already a human emotion. Magical Beast were shown have their own independency as seen with the shadow beast. The Queen herself had human genes as well and is considered to be a magical beast. It definitely makes sense why Youpi would developed such human emotions. His development was handled beautifully after all.

I believe by 360 you mean 180. 180 is change in character while 360 means a character going back to the way they were.

Also Killua was never able to fight stronger opponents before, every time he tried to he always ran away, only when Gon was in danger, he tried to overcome his habit but Illumi pop up telling him he couldn't do it which represented Killua's fear of Illumi as established in the end of the exam. Episode 20.

Killua was only able to find the needle when he overcame his fear through a lot of determination. Also it's not like when he took the needle he didn't have confidence problems anymore this is definitely seen during the invasion and it's planning. He also did run away from Youpi and was able to fight Pouf because Pouf was already in a weakened state after giving almost of his energy to him. I could talk more about his other development in the arc as well and ironically that has to do with the overly attached girlfriend comment you mentioned.