What I like in this scene it's the nervous tension inside everybody, especially Mike, Tom and Sonny. When I watch the film, I feel this tension too. It's one of those moments when our minds are racing: Fear of death (own death or a person you love), doubts about the decisions, getting distracted by second thoughts, physical needs and pain (Michael, for example wasn't able to eat anything), questioning the judgment of others. All this in the speed of light.
Tessio looks tensed too when he explains the box-and-chain-thing. Clemenza looks like a rock in the scene, shoveling Chinese food in his mouth with his dirty hands. He's trying to calm the rest of the group down. ("The gun'll be there." "He's playing pinochle with my men.") But when they all made up their minds, he can't hold the tension either, jumping up: "Let's move."
So my point is: Tom did have second thoughts, but also he was really worried about Michael. And that's how everybody else felt.