> I still don't understand how the National Association of Retailers enforce the 5-6% total commission.

Plaintiffs submitted some recordings in either this or a related case of an agent who said she wouldn’t even bother to show a house that didn’t have a guaranteed commission. The NAR is not so stupid as to have a “you must sign here that you will only show houses with a commission” form.

>> the incidence question … is a semantic one

Definitely not semantics. This relates to the elasticity of demand and supply in the market. The incidence can be on the buyer OR the seller OR shared. Depends on the good and the demand.