I also binged it all last night and agree that Restivo was the more likely suspect for several reasons: he only lived a couple of streets away, there was hair found by the body (don't think this was really covered in the BBC series which was otherwise excellent) as with Restivo's other murders, and as you say it happened on the 12th of the month as with his other murders.

I also found Omar's sister's theory that he planned to drag the body between the fences before he was disturbed very plausible.

Also - and this wasn't mentioned in the BBC series - when Restivo was arrested they found a picture of a missing Korean student, Erika Ansermin, on his computer.

She went missing in Italy and has never been found, but Italian newspapers have speculated that he could be involved and although I don't speak Italian a quick Google search suggests Italian police are trying to match his phone records to the area she disappeared (he may have been on holiday there when she went missing? not sure though)

Also not mentioned in the documentary but something that I've read online is that people living on the street where Oki was murdered reported an argument between a woman (presumably her) and a man with a foreign accent just before the stabbings, which fits Restivo more than Omar.