Reiri might lack a formal education, but she's not stupid, she's figuring things out on her own. Let's see what kind of trouble she'll cause.

needs more

Stupid. If you have someone as close as a body-double you'd damn better keep them loyal; otherwise you are in real trouble.

Historic situation is a bit simplified here. The castle probably wasn't reinforced because the Takeda were involved in the Uesugi succession crisis, and the Hojo going over to the Oda alliance. Likewise, they were themselves negotiating peace with Oda and might not have wanted to provoke them.

how did they think they could keep his body double in the dark?

There's so much strategy here, so good

T-t-time skip!?

ahh that was smart