(Stripe) CRM_Core_Payment_Manual::setAPIParams() Fatal Error
Gonna mark this as answered, for now, as I've logged it as an extension issue. 

The core of the problem, just FYI, seems to be that the auto-webhook process is failing (it appears to work, and a webhook does indeed appear on the Stripe dashboard, but the errors persist and I'm hoping that the real weirdness (the additional $200) is at least related to this.