Hello Renzo,
The name "Yamane Kazuo" is a fairly common name, so it would be a big coincidence if the person named in the article were the same person who signed the flag. Also, I don't think the kanji in your printed example is correct. I'd like to see a clearer photo of that flag, but it looks like it might be 山根和雄 (also pronounced Yamane Kazuo), but it could also be 山根正雄 (Yamane Masao). Also, be aware that unless we knew for sure how the person in the article spelled their name, it would be hard to link the two with any certainty. There are several ways to write the name Kazuo, so you'd need to know how the name is spelled to match up the names.
For the rising sun flag, all I know is that authentic rising sun flags from the war era are super rare, as most army soldiers carried Japanese flags, with just the red sun disk without the rays. Then, after the war, the "rising sun" flags with the rays became popular as souvenirs, so a ton of these were fabricated to sell to GIs who wanted such souvenirs. The guys at Warrelics.com have more details on the various flags. Find the subsection on Japanese war memorabilia. It is probably the most active site for such items on the web.