at the dons masacre, neri says mikey this way, then vincent and mike escape. wheres neri, why would they leave without him?

Good question, but I've never run across a reasonable explanation.

Also, why would Michael keep Neri around since he would be a constant reminder of his fratricide which he has come to deeply regret and which haunts him?

Hes there rember it shows him later back at the house...."Just when i thought I was out they pulled me back in" rember the reason he didn't leave with them is Mike was more important Neri knew that he found his own escape.

i noticed neri wasnt with mike n vincent when they left or broke into the car but he was in the next scene at the house .
Their is also the scene were Vincent bites Zazas ear and neri had left the room before Vincent enters but then he appears out of nowere to pull Vincent off Zaza sorry for goin a lil off topic

Neri does come and go pretty strangely in Part III -- I will agree with that. And someone does bring up an excellent question about Neri still being around -- especially if he reminds Michael of killing Fredo.

I think that up until Part III, Michael has only started to regret killing Fredo, so he wouldn't have done away with Neri quite yet. By the time Mary is dead on the opera steps, Michael has realized the entirity of his sins and at that point would be finished with everyone he ever did business with.