McCalman is the incumbent.
There are very few decent 8s.
And of those props you listed, Slipper, Alexander and Kepu are loosehead props who can play tighthead.
They are not tighthead props.
Id suggest that Saia Fa'ainga is the luckiest player picked. Hanson from QLD is better than him, and Nathan Charles is a tight playing hooker who we need, who IMO is the 3rd best hooker in the country.
Id rate Fa'ainga as maybe the 6th best.
Edit: And Elsom was possibly the best blindside in the world around 2005/6.
It was when he went to Ireland that he really went to pieces.
Id suggest he is one of the best blindsides the Wallabies have had in the pro era.
As Slim stated above, Kepu may have started at LHP but he is infinitely better at TH (indeed he may have only really been at LH as Baxter was the Tahs TH for many years. Slipper is a TH who plays loose and Alexander may be a backrower who plays prop (though he has been scrummaging much better this year under White)
Very few decent 8's? Vaea hasn't been given a go in a Wallabies squad, Mowen was a standout last year and is about the best this year. Fotu definitely deserves a chance. I would suggest that Mowen would also enhance the Walabies lineout out of sight. Imagine if you would two good second rowers, Higginbum/Denis at 6 and Mowen at 8. That is 4 genuine jumpers, something that Oz hasn't really had for many years and that would be a truly balanced backrow with Pocock. McCalman is the new Brown, impossible to move from the position even when others deserve the position much more by simple form and I would add potential.
I am not a fan of S. Fainga'a but you haven't been watching the games very closely if you think he's 6th best hooker in Oz. 4th I can see an argument, 3rd as current performance. Moore is so far ahead of the field it should be embarrassing the others.
The fact that you'd rate Elsom ahead of Cockbain and Melon Finnegan in the pro era says it all for me. Elsom would never have been rated above Juan Smith at any stage and there have been quite a few others who would have better better when he actually played well for Oz. It was in fact when he came back from Ireland that he never found form again and never deserved selection to be pedantic.