Right, finally resolved this issue to the point I can play the game, by getting a new keyboard.
Video evidence:
Anyhow, what I’ve finally realised and confirmed today, is that 1 or twice a shard in SR left/right click will be lost and become non-responsive, usually it resolves quickly, but sometimes the only way to solve it is to hit escape. This can be further mitigated by changing the USB port the mouse is in when the game boots, increasing priority in task manager, shifting it off cores 0 and 1 and setting compatibility to Windows 7.
However, per a fuck ton of testing over the last week, this will not resolve it too 100%. Because as I found out today, for whatever reason the Steelseries Apex 7 keyboard magnifies this issue massively and none of the methods above solved it. Just made it occur less often. But by the third set of SR levels, usually these would become barely effective and it was better to quit to desktop and restart the game.
Said testing involved uninstalling drivers, reinstalling them, trying different polling settings and using the repair.exe and deleting and remaking keybindings.txt. And a lot of swearing of course. Plus some percussive maintenance of the mouse, which it survived, so the Chakram gets the angry-bastard seal of approval.
Although I ended up spending over NZ$200 getting the parts to test this, namely a new keyboard and a 2nd hand Logitech G500s mouse.
Can say issue is not present in any other game I’ve played since mid July 2023 and is only really noticeable with channelling builds and ones heavily reliant on Attack Damage Converted to Health. Nor did I ever notice it with my venerable G9 (needs a new flex cable damnit) or the G502 Hero I used for 2 years until the middle mouse button died just after warranty. Which is why I no longer recommend Logitech mice 
As for why this occurring - I put it down to how ASUS had to implement the joystick this mouse has on it’s left side. Not the most useful thing due it’s poor positioning, but eventually I’ll get a 3d printed extension for it. Anyhow, this makes the mouse count as a controller in Steam. So what I hypothesise is happening is that GD recognises the mouse briefly as a controller and the game thus expects signals from a controller’s triggers rather than those from a mouse.
Not being able to dig into the code, this is the best I can do.
Which it’s why I will now tag @Zantai Mainly because he ignored my DM about this 
/tut-tut
Because this issue really does need a solution from Crate. As I suspect ASUS is going to keep that joystick as a thing, because it can be bound as 4 different keys or to macros, which is convenient to some players. Especially those with freakishly long thumbs. But I am happy to help be the guinea pig for helping to solve this. Does help I can’t work at present lawl.
As for the Apex 7, already talking to SteelSeries Support about that, which the keyboard change having worked, means they’ve got an issue. Should be an easy-ish fix though in theory, since it due to them bundling some mouse stuff in with the keyboard. Which I need to go make a post on and also link to this post on the ASUS forum to update them.
Right, more stuff to type 
Least the NZXT is pretty solid to type on though, much better than I was expecting for a sub NZ$200 mechanical keyboard.
Edit - fluffing grammer is hards when you’re on your third night of broken sleep