That would likely be because of the way they have all the DLC set up, with it all being purchasable in game they probably account lock it.
I believe I've run into the same situation with The Crew, where the game itself works on other accounts but none of the DLC does.
If you bought the game digitally then you don't need to have IW installed to play MWR, because the game license is attached to your account. It's currently impossible to own MWR without IW so no need to require IW to be on your system.
I think someone is trying to screw you over or doesn't know what they're actually talking about in regards to what they're selling you.
If you log in to a "Guest Account" on the PS4 it means just a blank user without a PSN account attached to it. That shouldn't matter in regards to playing...
Based on what people are saying from the Early Access thing the campaign might be extremely short, so I'd say only get it if you liked the beta/like Battlefield Online in general
Apparently they aren't sending out review copies to anyone, so take that how you will. There are review copies, they just haven't went out yet. Don't know what embargo is yet.
Honestly the videos and info from the two guys posting on reddit with the game did more for me than any review would...