Legendary pokemons would be rendered no value if that happens about the second suggestion its a little complicated because as what others said you can change your birth date to gain the presents.

The legendary idea, 0/10. Would majorly decrease value.
And the b-day would be a good idea, since eitherway they take their b-day or not it only occurs once a year. I would suggest not able to change b-day and linked accounts must match or be auto-matched. 7/10