Spoiler: 738
I agree that Tomoaki is suspicious precisely because he got attacked by the demon dog (AND DID NOT DIE!) He could have brought the cabin to the detectives' attention because he had prepared the cabin in advance and planned on the detectives going to search for the dog after the "attack" on Ran and Kazuha (it would have worked no matter who was feeding Hachi, as long as the demon dog could make an appearance at the house.)
Also, at this point, Miyuki is too obvious to be the culprit. However, if she is not the culprit, there's still some explaining that needs to happen regarding the Eight Dog Chronicles, and the sharing of kanji radicals between the old woman and Miyuki. Obviously it doesn't mean she is the culprit just because of her name, but it has to tie in somehow.
Also, at this point, Miyuki is too obvious to be the culprit. However, if she is not the culprit, there's still some explaining that needs to happen regarding the Eight Dog Chronicles, and the sharing of kanji radicals between the old woman and Miyuki. Obviously it doesn't mean she is the culprit just because of her name, but it has to tie in somehow.



