I think Tom had foreknowledge of Fredo's fate. I think his motivation in going along wasn't so much direct fear of what Michael would do to him. I think it's fear of loss of the connection to Vito, which Tom maintains by serving Michael.
It looked like Michael had forgiven Fredo, the brothers hugging each other at Mama's funeral after Connie's tearful plea until we saw Michael's chilling kill order look at Neri and Neri's subtle nod
Tom could have thought the same believing Fredo had gotten a pass Tom wouldn't have known with Mama Corleone's death Fredo's time was up
Same with Connie otherwise Connie would not have taken Anthony off the boat and let Fredo go fishing with Neri!
Still Michael could have easily continued the same arrangements of keeping Fredo under watch or similar until Mama Corleone's natural death. He had the money and the resources
Besides
if Mama had lived longer....
