How would adverts dictate these rules, what can and cannot be said in a review?
There are no fixed rules but restaurant owners can react rather badly to critical comments, even if the overall review is positive, so negative comments are simply not made.
Are the reviews written by someone working for the guidebook/magazine?
Do any adverts for local restaurants come through regular mail?
No. But menus for pizza delivery places are often left on one's door.
Do restaurants in Poland have mailing lists you can sign up for to get notification for daily deals and specials?
I'm not aware of any which do that (although there are a couple of places which send occasional emails).