@Acolytus The trees are just something that family does. There's no real confirmation that either of them died. If you look at previous chapters, when the FMC first arrives at the house, she (actually it's her sister) is shown "her tree" that they have planted for her.
@UncleBenny Also, in Chapter 11, we see the FMC's sister with her husband, who looks nearly identical to Toyokichi (the buzz cut guy). Remembering the fact that FMC's sister said she'd marry him once she's older, you can put two and two together and assume that he survived the war. Now Isaji on the other hand, we really don't, but based on the sadness of this series, I'm going to go with yes.