Did any of the paint even get damaged? I don't think a decal would be necessary at all there.

If you wax and put down mylar, that would at least prevent it from getting worse.

Black is at least easy to paint. Just add a little white or gray to match the black on the playfield (since the paint right out of the bottle would not be an exact match).

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

I get your fear there, but there's a good thing with this super glue: Acetone will remove it if you happen to spill a drop of it or smear it on a playfield (ask me how I know, LOL), and I've never had this particular super glue dry with a haze. It's always been crystal clear. Supposedly nail polish remover works as well to remove the glue, but I never had to use it. I bought a can of Acetone at my local Lowe's, and keep it on hand for just such a mishap, and I haven't had any damage to the surrounding clear coat from using the acetone, but then again, I wasn't soaking the area down. I applied the acetone sparingly to a rag and worked only the spot I needed to, in order to remove the excess super glue. Just beware the fumes of acetone. You want to have decent airflow around you if you have to use it. It stinks and I'm sure the vapors aren't good to breath.

Careful with acetone. If the clear has warn away, it will remove the paint completely.

I'll second the CA glue. Polishes up wonderfully.

This is what I use.
https://www.amazon.com/Bob-Smith-Insta-Cure-Super-Thin/dp/B001NI4JWI