Will Neil Arrest Avni

Neil being an honest cop n for him duty comes first

Neil didn't arrest Avni for case 123 neither he will do it for the accident. He will infact ensure that Avni is not held responsible for the accident. Like Daya said Neil ki jaan Avni mein basi hai aur apni jaan to sabko pyaari hoti hai

If a situation comes up where he has to help both Juhi n Avni who is he going to help first or get out of the situation first - I wish this happens...

Neil won't arrest Avni, he didn't for case 123 and he won't for this case. Neil is investigating this though and is wisely getting a DNA test done on the body. Neil and Avni both believe Raghu Pandit is involved in all this and technically they are not wrong, GM is involved but there are more players and hopefully they will learn this fact soon. Neela is getting suspicious of Amol so I am hoping that leads to his downfall since Ali cannot reveal this truth to Avni and Neil.

Technically speaking in hit and run case investigation happens on the extent of negligence and fault of person driving the car and then arrest is made depending on it. Here yes that min dies after hitting Avni's car but is she at fault. No it won't be because for his body to be mutilated the car need to have run over him and it should have crashed against it badly. But if none of it is in evidence or play then clearly driver won't be at fault. So currently Neil has to follow logical course of investigation which he is doing.