Hi stephane,
Instead of using a butt-plate, I use "floating" sidemount.
I have a 12 ltr carbondive with Nitrox and a S80 with trimix 15/50.
Both have a floating ass !
I have installed two new D ring on my belt but in the front and not on the side. I clip the carabiners on those drings and the tanks float side mounted.
The acces to O2 and dil add buttons is very easy, my arms are above the bottles.
For the bungees, I use the free hole of the dive rite harness just below the two bolts (see your picture 3). Double bungee. I just put a small rope linked to plastic D ring of the dive rite harness to be able to find them.
I did several dives like that, very nice.
Acces to regulators are very easy, they stand on the bottom face of the tank and you find them right away.
In caves, you have to be carefull in some restrictions, not to grab nasty stuff inthere. I have still to work out this point.
Some pictures of a test dive in a Belgium mine (35 m deep )
I have never tried an Armadillo system so I cannot compare
Take care
Bruno