Not a TIPS, but since the anime cut out Oishi's interrogation with Shion, here's a brief rundown of what was said during the interview:
A. Houjou Satoshi disappeared.
B. A huge amount of money was withdrawn from his postal-bank account on the day that he disappeared.
C. The teddy bear at the store was gone. However, the store cannot recall whether it was Satoshi who bought it or not (The anime portrays the shop keeper as a young man. In the game, the shop keeper was an elderly man with signs of dementia.)
D. If Satoshi acted accordingly, he would've:
- Withdrew all the money
- Went to the toy store to buy the big teddy bear for Satoko's birthday present
- Since the present is huge and thinking how Satoshi really was looking foward to giving Satoko her present, he should've gone straight home.
E. Steps 1 → 2 had to happen. Satoshi was working himself to death to surprise Satoko. He would never have thought to just abandon Satoko and take off. Since the teddy bear at the store was gone, Satoshi must've bought it.
F. Hence, if Satoshi had disappeared, it would've been from Steps 2 → 3
G. Did Satoshi disappear? One hope is that there was a report at a train station in Nagoya that a kid fitting Satoshi's description came up and asked whether they have his wallet at the lost and found. When asked to write down his name, he started to write "Houjou," but stopped immediately, crumpled up the paper, and wrote a different name instead. Another station attendant at the same station saw a person fitting the exact same description jumping on the Shinkansen headbound for Tokyo. It is a very slim hope, but Satoshi may still be alive, somewhere underneath these skies.
H. But why? Why would Satoshi decide to leave Hinamizawa, leaving behind his sister that he cared for so much?
explained to Shion:
A. Satoshi and Satoko were abused by their foster parents. It is inevitable that Satoshi might've had a breaking point. So let's say (Oishi stresses this as a theory, since he has no proof due to Shion's alibi) that Satoshi did kill Aunt Houjou Tamae.
B. However, because of Shion's alibi, Satoshi was exonerated.
C. Unfortunately the Sonozaki head, was highly displeased that a member of the Sonozaki would fall in love with a member of the traitor, Houjou.
D. While Shion was able to redeem herself by breaking off her nails, Oryou may not have been satisfied. Shion did declare her love for Satoshi openly and proudly at the torture chamber. But once again, this is the Sonozakis. They must've known that Satoshi was the real killer of Aunt Houjou. It was unacceptable for a member of the Sonozaki to be in love with a troublemaker killer. So Oryou may have decided that she needed to teach a lesson once and for all so that Shion can forget about Satoshi.
E. Oryou must've ordered the (literal) disappearance of Satoshi.
F. However, Mion must've compromised to spare Satoshi.
G. So, what would the Sonozaki's do to make Satoshi "disappear?"
H. Now what if, (and Oishi stresses this again) the teddy bear was bought before
Satoshi arrived at the toy store?
I. Yes. That's a highly remote possibility. The teddy bear sat there all this time, while Satoshi eagerly looked at the window as he made and saved money to buy that bear for Satoko. Satoshi even reserved that bear with Shion's advice. What are the chances of the bear being bought on the day Satoshi was supposed to buy it?
J. But we're talking about the Sonozakis here. If the local yakuza ganged up to the store, dumped a wad of cash for the bear, would the toy store refuse because it was already "reserved?"
K. Now the teddy bear was gone. Satoshi, excited to buy the bear, looks into the window and sees......nothing. All his toil and sweat...for nothing. All he wanted to do was make Satoko happy. But, someone else bought the teddy bear. What a despair. Poor, poor Satoshi. What must it been like for him? His heart must've been ripped to shreds. He has never cried, but one can imagine him crying and screaming when he saw the teddy bear gone. Now all he has is a stash of cash in his hands that is practically useless to him...
L. Sonozakis comes up to him: look, you have nothing - you don't belong here. But since we're kind, we're gonna let you live. But, you can't come back to Hinamizawa no more. We've set up a safe house for you in somewhere far away. Why don't you use that cash to get there? We'll promise to take of Satoko. The condition is - you gotta go.
M. And that's how Satoshi left...in despair, leaving behind Satoko. With no present to give, being a killer, he must've felt that he had no way to show his face to Satoko. With Sonozaki's pity, he straddled to Okinomiya Station...to Nagoya...and onward to Tokyo.