Update: After another lengthy call from Apple yesterday, since I had no documents on this machine yet, they asked me to wipe the SSD and reinstall via the Recovery Partition. That in itself took much longer than I expected (1.5hour and I have a fast broadband connection) but I'm now back into a factory fresh OS X and am not installing anything at all for now.
Last time I had just Adobe Lightroom CC and Adobe Photoshop CC installed along with the Adobe Desktop Client and CC client that come with it and I had signed into iCloud. I didn't sign in to iCloud and I've installed no software and for the last 24 hours I've not seen a single graphics glitch, not even the repeatable split second one during login.
I'm continuing to use this as a (very expensive) web only device as a control test. Safari use only, no plugins, no App Store installs, no Adobe product installation at all, nothing. I'll report back again tomorrow and then install a few Apple apps, log in to iCloud, sync with Photos app, etc.
If it runs fine without anything installed then, to me at least, that's fairly reassuring that it's perhaps software related not hardware. Maybe it is Adobe Desktop Client that is causing the issues somehow? I know most people reporting the glitches were using Premier or media encoder at the time, unlike me who had no Adobe apps open, but there is always the Adobe Desktop Client running in the background upon login.
Before anybody says it, yes I know this machine isn't meant to be just a web client and I know that nobody in their right mind would buy a machine this powerful/expensive (subjective) to not install 3rd party apps but before we all start thinking the worst and blaming hardware failure, I'm leaning towards software incompatibility issues at the moment.
I'm very tempted to just install Adobe Lightroom CC and Photoshop CC again like last time and see if that provokes a graphics glitch but I feel I should do some more control tests on a fresh installation for another day maybe to reassure myself there's no issues as it stands.