Harness arrived today, I did what Nuje indicated and fishing it was a breeze (probably less than 10min). Once I removed the side panel I pushed the cable down into the passenger footwell. Then, from the footwell, I negotiated a hanger wire up (there are a couple of openings, I guess I was lucky to find "the right one" on my first try) until the tip showed up through a hole to the left of the box the headunit would slide into. Of course, that wasn't so lucky, as you can't connect the cables to the headunit in that situation (I mean, I haven't even tried, because it appeared that the cables would be caught in the very tight space between the casing and the headunit to the left as you push it back in). There is a hole in the back of the casing as well, so I pulled the hanger wire back a bit, twisted it slightly and pushed upwards again, and this time "Bingo!", it came straight through that one. Then closed the glovebox, cut the excess wire in the footwell, bent that end a bit into a tiny hook, hooked the cables and used electrical tape to tighten the bond. Fishing them up was a breeze, connections likewise etc (as described in George Ab's original post on golfmk7), then pushed the headunit in until locked. I actually spent more time (maybe about 20 min or so) trying to seat the cubby back in properly (it would click in but spring back out, until I figured I had to slide my hand in and push against its back panel until it locked into a round opening inside the dash, then it locked perfectly in front too).
I had, of course, spent some time prior to all that connecting the cables to the headunit and check that the harness is working. Then disconnected and did the above.
I now have CarPlay working (can make and receive calls, sound is perfect). Everything is functional (rearview camera, CarPlay, car stats etc) except Sirius XM (says I'm not subscribed, I guess I need to call them to change the radio code) and there is no sound for radio or media (other than phone calls). I contacted Jack and hope to have everything done soon!