How untangle two Whatsapp accounts created with the same numer? First, I installed Whatsapp on my Alcatel phone, with my whatsapp account ("son").

For my old mother, I bought her a big screen, cheap second hand Samsung Galaxy, (poor eyesight). That Samsung is blocked by the phone company (long history, don't ask); it can't make/receive calls. 

I installed Whatsapp (via Wi-Fi) on her phone using my own phone number (stupid mistake) to get the audio check code. She got her own account ("mother"). It works almost fine, but I ("son") can't add new contacts because invite responses goes to "mother" account.

I changed phone number in "mother" whatsapp to her real working Nokia basic phone, but still doesn't work.

Short of deleting "son" and "mother" Whatsapp accounts, or buying a new SIM card for me, can you think another solution? I live 4 hours away from "mother". I don't want to lose 2 years of messages and contacts.
You can save a backup of all messages you have, then going through settings - account you can either: delete the account then activate it again another time using the correct number for each phone. or use "change my number" and reactivate the account with the wrong number to the correct one