I went through the last handful of chapters. In Chapter 1041, Momo says he does know why he can't die, but he doesn't understand why Roger laughed at Laugh Tale and suggests that the two are tied in some way. This leads to him questioning why Oden wanted to open Wano now since there's so much danger to the citizens and that he wants to protect the people of Wano at all costs, even if it goes against what his father wanted. He then asks Yamato if he's a coward for thinking that way.
I don't see why that means Momo needs to leave with the Straw Hats, since his concern is only that Oden wanted Wano opened now for Joyboy's return and why opening it is considered more important than the risk to the people of Wano. Luffy is the long-awaited Joyboy, and once Kaido is dealt with and Momo opens the borders, that's it. At that point, whether or not Momo understands will be moot.