I would get some water thin superglue and put it in a syringe (if available). I would then inject a tiny amount into the crack to see if capillary action (wicking) would allow the superglue to flow into the air gaps under the insert's clear coat, which would remove the white area from beneath the blue insert's clear coat and restore the blue color. I would wait for that to dry, and inject another thin layer into the crack. This should seal the air gaps under the insert's clear coat.
At this time, if you're positive the prior applications of super glue sealed the area under the insert's clear coat, you could then use some black acrylic paint to touch up the key line. Note: If you didn't get a complete seal on the air gap under the insert's clear coat, the black paint could wick it's way into that air gap and make a noticeable black incursion under that clear coat, which you'd be stuck with and you'd be bummed with, so beware of that.
At this point, the crack should still be slightly below the playfield level (at least that's the goal). I've then filled the rest of the gap with water thin super glue and sanded it flat with super fine sand paper and polished the area back to where you wouldn't know there was an issue. I suppose you could use clear nail polish or a touch of auto clear for that final step as well.
That's what I'd do, and that's what I've had luck with several times in the past. As always, your mileage may vary, and my past success is no indication of future success, LOL, but it worked for me.
FYI: I always keep some of the blue label water thin super glue on hand that's found at this link, just for this purpose: https://www.hobbylinc.com/ca-super-glue