@jwrm
Unfortunately I think that you are going to have to accept that motherboard now damaged as here:https://forum.fairphone.com/t/fp2-is-dead-solid-red-notification-led-for-some-minutes/21328
As am in UK could have sent you fully charged battery to try, but think that it would be a waste of effort. As motherboard most expensive component and can only be replaced by Fairphone for €200, suggest you look out for one being sold by existing user once their FP3 arrives if you still want to stay with Fairphone.