What are our options?

Trim

Sorry your photo is blurry so I can’t see much but trim sounds like the only option. How high are the counters? Why the spacer? That metal bracket on the island looks odd to me. What is it attached to?

Did you have a larger mitered or built-up edge on your previous counters?

Looks like you did. Why was that not discussed?

You got the order of operations wrong. The old counter should have been removed before the cabinets were painted.

The solution now is to remove & replace the corner trim and call the painter back.

You can ask the painter to come back, but that is going to be a minimum of a $1K charge. After a project is done, none of that leftover paint is kept beyond the token amount given to you as a touch up. He has to buy a whole new gallon, and fit you into the schedule to sand and refinish that whole face frame, and zip wall it off while doing it. It would be cheaper if you just asked him to buy the matching paint, and brushed and rolled some cove trim yourself. Then cut and install it yourself, and touch up. That's still $200+ by the time shipping is counted. This is why painter chare 7-12K for doing the job.

That bracket isn't doing a thing to support a countertop. It has zilch to be attached to properly. That's the kind you attach to a pony wall. Seems like most of the issues here involve the countertop people not communicating or doing things properly.