1.Her father and Juko are siblings. I think you can tell from red hair color on both. Rumia is not related to them in anyway. As you can see her designed is changed in LSHRM. Also don't have surname for Hanabi yet as we are yet to meet her in LSHRM.
2.That's Junsa during her college days. It was suppose to be Rumia hinting that she was there even before Masuo's birth.
3.They both are same person. It will be more clear when Ryoko's gameplay sequence is available.
4.I think it's name for Masuo's blonde friend. I use haven't decided on his name yet.