Cain has a history of using other people for the dodgiest jobs. He used Ross first in October 2013 when Ross then showed up at Butlers Farm with a gunshot wound from a bungled job which Cain had told Ross would be "easy". The new spoilers also say that Cain asks Adam to help kill Robert but Cain ends up going to do it himself at the hospital.
I looked at the shooting episode (Sept 25th) again yesterday. After the shooting, Ross goes into Debbie's house a few doors away from the pub and tells her in a sort of surprised/shocked voice that "Robert's been shot", as if he'd only seen Robert laying on the ground at the back of the pub rather than having shot him himself. Just my impression really but we all know that Ross can be capable of anything. He also said that he'd "nearly" done something but changed his mind as he would end up in jail if he stayed around Emmerdale. I took this to mean that he was going to kill Pete at the farm but didn't, maybe because he thought about the long-term consequences, and decided to leave Emmerdale for now, if not forever.
I still can't think of any motive for Ross to shoot Robert other than on behalf of somebody else for money. If Cain or Ross was the shooter, I'm not sure if he used a new gun or the one from Home Farm (Lachlan's). I do think Lachlan had his gun with him in the village that evening and then threw it away as he walked home but whether he just was scared after having seen Robert shot by someone else and Lachlan just threw the unused gun into the field or Lachlan had used the gun to shoot Robert with is another matter.
One of the scenes we haven't been shown yet is who was inside the pub when Robert was shot. Maybe Cain or Ross (or both) were in there at the time? We were shown the inside of the cafe and know who was in there at the time.
I do wonder if Chas lured Robert to the back of the pub by texting him minutes before and maybe leading Robert into a trap with the shooter already in place and known by Chas but I'm not sure if Chas would be told about the details of the hit if it was Cain doing the shooting (or Ross was doing it for Cain). Cain wasn't seen in the village that night as far as I remember. Ross could have done the shooting for Chrissie or even Lawrence, of course, with money likely to be the main motive.
There are still many questions to answer.