If the choice of helmets all have a similar crash rating then the most important thing is the fit. Helmets are different shapes internally and heads are different shapes. If you get the wrong shape helmet for YOUR head, it will not be comfortable and you'll find it distracting.
I use a white Caberg Duke. This suits the shape of my head. It has a 5 star Sharp rating and many Shoei and Arai helmets don't make this standard.
I don't believe that's an accurate statement where specific test standards are concerned. First, every safety helmet offered by Shoei
is Snell Foundation certified. And it's common knowledge within the safety helmet industry that the Snell Foundation testing protocol utilizes a higher impact (anvil) test standard than that of the UK's Sharp standard.
... BTW: I think like 80-90% of helmets out there are OEMed by a handful of manufacturers. HJC and Z1R are supposed to be 2 of the biggest OEM's out there. Supposedly even AGV doesn't own it's own manufacturing facility and gives the specs to an OEM to make the helmets for them.
The term OEM as used in your post is a bit misleading... the makers of safety helmets are generally thought of as aftermarket manufacturer's, not OEM's. Whereas the term "OEM" (Original Equipment Manufacturer) is typically used to describe factory replacement parts & components sourced from the manufacturer of a vehicle.
What you're describing is commonly referred to in most industries is 're-branding'... in the case of helmets, Bilt
helmets for example, is a "store brand" helmet owned and sold exclusively by Cycle Gear. Cycle Gear obviously doesn't manufacture it's own helmets, and as you say, the Bilt brand could very well be made for Cycle Gear by a company like HJC or similar.
As for AGV, they are part of the Dainese brand group and like Shoei and Arai, are considered to be a top tier helmet brand. That said, I would find it hard to believe that AGV helmets would be made by a company like HJC, which is a brand known for it's lower price point helmets made in China, and therefore is not generally associated with top tier safety helmets such as AGV, Shoei, & Arai.