Is there a way to change the starting abilities of the player characters in the GBA ?

FF6Tools is much more complete to edit the GBA version, it surpass my old editor: https://everything8215.github.io/ff6tools/ff6tools.html

For spell names, it can maybe edit them, if not you can do it with a hex editor like Windhex with the correct text table loaded. Spell names offset should be on the ROM map on the wiki.