not sure who bred the first yellow bellies in captivity, like most morphs they are wild collected and then proven. i think amir was one of ( if not the) first to prove yellow bellies and ivories.
It is possible for people to have wc yellow bellies. Again people have found them in large shipments and proved them out. There will always be a lot of difference in a huge shipment of balls, sometimes nearing 50000 will be shipped at one time.
Alot of people think they have YB's but in fact alot of the time the are misrepresented animals from shonky dealers!!
check out the link below, great pointers
http://www.nextworldexotics.com/hgyb.htm