The flag with the writing on it says
大正四年元旦 Taishō yonnen gantan
ニッポン Nippon
恭賀新年 Kyōga shinnnen
1914, January 1st
Nippon/Japan
Happy New Year
I have no opinion on authenticity on that flag, other than I find it slightly weird to use katakana for "Nippon", and the lettering is very funny. I don't think I've ever seen a flag with "Happy New Year" written on it. It's obviously not something from the war. Whether it is really from 1914 or not, I have no idea. Is the funny lettering just a weird stylized writing, or does it indicate that a non-Japanese person wrote it... this is also something I don't know.
The sun ray flag... there are no clues on the flag that would tell us anything about its authenticity, unfortunately.