I really love this name but am not Welsh. I grew up on the borders of North Wales with lots of Welsh-named school friends though and they generally pronounced it "Beth-An" with the emphasis on the "An" though they were not generally Welsh either - certainly not Welsh speaking though they probably had Welsh backgrounds. I've since heard "Bethun", with the emphasis on the "Beth" and I read a confusing thread with people from South Wales disagreeing about the correct pronunciation. Is it optional/variable depending on accent/dialect or is one of the pronunciations definitely wrong?
We would probably use Beth on a day to day basis anyway so I am not massively bothered if different people pronounce it differently but I am lobbying dh to make Bethan the official name and my cause is not helped by being unsure of how it's pronounced.