"Only a comparatively small proportion of the total in Mr Laycock's list of invaded states actually formed an official part of the empire. The remainder have been included because the British were found to have achieved some sort of military presence in the territory – however transitory – either through force, the threat of force, negotiation or payment."
More: Nine out of ten countries have been invaded by Britain -- but the United States is catching up
Is the remainder in the above text an indefinite quantifier as a result have has been substituted for has? Is that an acceptable in American English?
"The remainder" here is a collection of states, hence plural, hence "have." This is good in AmE.