As title really, I want to run one USB cable from the cubby box area through and up to near the head unit, and then another audio cable from near the head unit down and through to the aux socket on the rear of the centre console.

How easy is it to get the centre fittings apart? On my P38 it was pretty straightforward, but that was held together with gaffer tape and spit for the most part....I assume the L322 is bit more complicated?

I'll also need to get the head unit out and possibly some of the panels lower down, how do they come off?

Once I've done all that, I need switched and constant 12v near the head unit....are they available in the wiring behind it?

'easy' and 'near' are all somwhat relative

There's a couple of videos on the Powerful site that may help (or not) -
https://www.powerfuluk.com/video

Power is probably best taken from the fuse box behind the glovebox using piggy back fuses - search for the dash cam installs for pics - like this one

Feeding cables can be assisted by using net curtain wire or fibreglass cable rods.