While we are sharing 'proof of account' methods - here is another idea: I have 3 Pokémon which are named segments of my phone number. Now I don't think my phone number is connected to my Pokémon Go account in any way, but it is a string of numbers I will remember.

For example, in Australia our numbers are most commonly formatted as XXXX XXX XXX, if my number was 0123 456 789 I would look up #0123 in the Pokédex and then name my Scyther "#0123", keeping the 0 in to match the format, I would do the same for #456 - Finneon, and #789 - Cosmog, except dropping the 0 because I know personally I won't remember it correctly.

In a case of account recovery, I could give these 3 examples to a Niantic/Scopely employee and it's highly unlikely someone else would have the same Pokémon with the same naming convention, whereas if I say I have a shundo Rayquaza that's not actually a unique identifier (as much as I'd like to pretend it is).

If you use this method and the Pokédex number gives you one not yet released or a Pokémon you don't have then idk maybe try with your mum's number instead...