Bob Keeshan and Lee Marvin both served in the Marines, but never together.
Marvin was wounded at the battle of Saipan, two months prior to Iwo Jima. He received the Purple Heart, not the Navy Cross, for his injuries. There is no record of him serving at Iwo Jima.
Keeshan enlisted in the Marines in June, 1945 - four months after Iwo Jima. In fact, he never saw action during the war. He was never awarded the Navy Cross.
I have a feeling the Fred Rogers stuff is equally bullshit as well as I can find no record of it anywhere. I did find that he wasn't a sniper and never served in the military.