I upgraded my setup to the latest MacBook Pro (the i5 processor started to struggle with some of my music creation plugins) and can gives some updates on my previous posts over the last 2 months or so. Basically all glitches were due to the late 2015 iMac, especially its graphics card. All is fine now:
No pink letter rendering in Chrome
No corrupted cursor when waking up from sleep
Now it is just down to small things: a colour picker, screen shots, and that CMD vs CTRL issue.
Cover image via WikiPedia