"so they would have to be selling HD broadcast back to Foxtel or BeIn" - beIN Sports is broadcast in standard definition, but as to your point, the rumours I've heard suggest Optus are keen to keep the broadcast rights to themselves.
Yes, it's credible. My source asked me not to quote them.
It's true that many of the advertisements are for South-East Asian nations. I also wonder if the fact that the Premier League is so popular in Singapore may have influenced Singtel's decision to pursue the rights in Australia through Optus?