my pinephone battery is dead :/

[Alho]

Pretty much what the title said, dead as in 0v on the multimeter

(...)

question number 2, shouldn't this have triggered some sort of overdischarge protection circuitry?

Yes, it for sure have triggered the protection circuit inside the battery. It happened for me with PmOS  running om it, until the battery cut it off.
The battery can be charged again, but it only charges with about 100mA in the beginning, so just be patient.