Is it normal for pulse oxygen level to gradually drop from 99% to 80% over several hours during a basilar migraine attack? or are there other reasons causing the drop? is o2 a better treatment than triptan in this case?

I'm dubious of that very low Oxygen saturation. It's possibly falsely low because your peripheral circulation in your fingers was vasoconstricted.resulting in poor perfusion of blood to the fingers. Triptans can cause vasoconstriction. I would expect you to have symptoms if O2 sat was truly 80%.

No. Be certain your device is working and maybe using another finger. Do your fingers turn blue.