Did any significant collaborations or mentorships occur between the prominent 'John' artists at Marvel Comics?

Yes, there was a notable mentorship. John Buscema, often revered as a master, played a significant role in guiding and teaching other artists, including a young John Romita Jr. Buscema's insights into anatomy, storytelling, and panel composition were shared with up-and-coming talent in the Marvel bullpen. While John Romita Sr. and Buscema were peers and friends, their relationship was more one of mutual respect and camaraderie rather than direct mentorship, though they undoubtedly influenced each other through their shared work environment and contributions to the Marvel style.