Is it possible to get my US citizenship because my Mother is a born US citizen?
I think there is an age limit involved. I'm Canadian born, naturalized US citizen. For my circumstances (opposite of yours) I believe you have to submit papers when your child is a minor (under 18), though perhaps there are loopholes to that. Maybe it's the same for both countries?
I wonder if it would be worth searching for immigration lawyers who deal with child abductions and Hague Treaty protections. They may have some ideas about how to mitigate the citizenship problem while your custody case is pending. A regular immigration lawyer may not deal with cases like yours very often whereas an immigration lawyer that deals with high-conflict custody abductions may have some creative solutions or workarounds.