"That would have explained separation of APU. To me it looks that immediate shockwave area right after the RPB is not damaged. Skin is not even peeled. APU firewall is not visibly pressed inwards by shockwave which separated it from the rest of fuselage."
The RPB is forward of the APU firewall (that we are seeing still attached to the tailcone) by several feet. The support structure for the HS and VS is in this area between the firewall and RPB.