Can the gap between the panel and fingerboard of classical guitar be repaired?

Use good white glue or slow-drying 3-second glue (you're right about 3 seconds, but there are quick-drying and slow-drying ones)

Prepare a c-clip.

Pour the glue into the seam, put it into the ring hole with a C-clip and clamp it up and down.

White glue takes at least 4 hours.

3 seconds glue should wait at least 10 minutes.

I wish you success!

Hold on. number ....

What I do is that you must hope that you will succeed.

This is how they are repaired in the factory.