If the Borlase drawing is inaccurate we are gubbed .If it is accurate then what you have a is a typical tripod portal dolmen i.e. the pointed backstone supporting the capstone either relatively flat or angle upwards from that stone e.g. ,http://www.themodernantiquarian.com/site/1055/legananny.html
http://www.themodernantiquarian.com/site/116/llechydrybedd.html
http://www.themodernantiquarian.com/site/117/pentre_ifan.html

The addition of side stones aren't necessary for support hence the sloping side doesn't matter ,although they do allow for ingress if the cairn is high enough .Trethevy ,rear of Poulnabrone have them . As for "Why would people with not a lot of free time on their hands find or fashion tapered stones and then balance a 10 ton capstone on what in comparison is a needle? " Who said they didn't have a lot of free time ? and even if they didn't ,we don't have a clue why they built these things , it's hardly utilitarian .If you want to dispose of of a few bones or lithics there are easier ways to go about doing it .If some sort of answer is needed then fwiw conspicuous construction , showing off .