On the beta 5 versus secretly beta 6 question, the good news is we will actually be able to see it.
The label Apple sticks on the developer portal is marketing. The build string is the honest record. Beta 4 was 24A5390f, so the seed number is 5390 and the f is the compile count. Those two things move independently and they tell you different things. The letter tells you how much grief that particular build gave them on the way out the door. The number tells you where it sits in the lineage.
So if DB5 lands as 24A54 something with a low letter, that is a completely ordinary next step and the delay was just a wobble. If the seed number jumps a long way past that, mikelets456 is probably onto something and a build got binned somewhere along the way.
Worth reading the string before reading the changelog, either way.