Thank you for a speedy reply!
Soul-crushing dread averted, the issue is resolved! I couldn't go in to the plugin manager or the update core settings because my RMMZ editor literally wouldn't open at all any more, but the issue was I had to right click it in the task bar and maximise it because for some reason its windowed version was invisible (or reduced to a single pixel or something?). I'm up and running again and no longer freaking out :)
I'm not sure if that's a fault with the plugin, it's likely more to do with the fact I dual monitor with two different resolutions (1920x1080 and 3840x2160) and I've had the odd bug before but never like this. But I'm sure it's a weirdness on my end anyway.
I've updated the core script and ran the game with the plugin and didn't get a crash this time! So that's a relief!
This might be a silly question now at this point but perhaps I'm overlooking something simple -I've opened the menu and pressed F9 and I don't get anything popping up to edit the UI. Am I forgetting something? I've disabled VisuStella MainMenuCore to prevent conflicts (though I still have other VisuStella plugins, incl. Core).
I read through the topic and saw that UICustom folder not appearing can be an issue too, but it's there, and has 7 files in it (bootstrap x2, common, index x3, jQuery)