Given: phone will neither charge nor fully power on. (Tries to if you force it, but stops)
Given: iCloud-related Apple ID for this phone is unknown, but would not be unheard of. We are the legal owners of this phone. If we knew the Apple ID, we could recover the password.
Given: the phone worked properly before getting stuck in this condition.
Assumption: we know the screen pass code for this phone, if one was set.
Now, the question: If I were to remove the battery and install a new battery, and the phone then booted fully, would I need any more than the screen lock pass code to re-enter the phone? Would the R&R of the battery do a deeper lockout of the phone, or would it be the same as swapping out a dead laptop battery?