Shazam belongs to DC Comics. DC has developed the character and its related storylines over the years. The powers and origin of Shazam are integral parts of the DC universe.
Captain Marvel is Carol Danvers. She gained her powers through an interaction with alien technology and has become a powerful hero in the Marvel universe.
One idea could be a story where Captain America and Captain Marvel team up to face a new cosmic threat that's headed towards Earth. They have to combine their unique skills, with Captain America's strategic mind and combat experience on the ground, and Captain Marvel's incredible cosmic powers in space battles.
I don't think so. The concept of 'strongest' is relative in the Marvel universe. Different heroes have their unique powers and moments of greatness, so it's not a straightforward answer to say Captain Marvel is the strongest.
Yes, Captain Marvel is definitely a significant character in Marvel Comics. She has her own storylines and plays an important role in the Marvel universe.