Lets say someone ends up with a severe poisoning that causes acute methemoglobinemia. can the hypoxia caused by the methemoglobinemia result in brain damage if no medical assistance is provided? is it possible for the body to even survive on its own?

It depends on the severity. Yes, hypoxia due to methemohemoglobin can cause brain damage.

Typically, there are genetic causes for methemoglobinemia, causing susceptibility to medications, or foods, even topical meds. If indeed there is a family history, you might be at risk. Pulse oximeters can monitor oxygen levels if such a problem were present, but hypoxia untreated can be deadly.