Much respect for you and your quest to get this back to the family of the person to whom it was dedicated. Give Obon a bit of time - its easiest when they have a solid name, date, and location to work from. When they just have a name (and a rather common name at that) it requires a lot more legwork.
Having said that, you should also be aware that there are a lot of fakes out there, and that even the fakes were quite popular as souvenirs among the GIs. I can't tell if this one is a fake or not, but the main writing on the right side is pretty poorly executed, which always raises suspicions. It's as if its a mixture of writing styles - blocky in places, cursive in others, alignment all over the place, and then the last two characters of the line containing the person's name (道君) look really crappy. There's a vertical line missing from 君, which is inexplicable. The rest of the signatures look OK, but very odd that the main writing should look so amateurish.