belly heat 100% helps with snake digestion do some research on there digestive systemthis is the whole reason why snakes will seek a heat source after eating. or if you have ever seen a local reptile laying on a stone after night fall. they are taking advantage of the belly heat the stone is retaining. thats why UTH and flexwatt are so widely used under the enclosure
because back heat is MUCH easier to keep ambient temps up. Some will use both as well.
as for belly temps from back heat. you could be 100% correct they could achieve desired temperature from it and they usually do. but there is also the possibility of a bad setup where the bottom of the tub becomes a source of heat loss in which case belly temp wouldn't be the same as the ambient or back temp. there are allot of dynamics to take into factor, it can sway either way.