You ask what a literary foil is, which characters serve as foils in the text, and what their relationships with Edna might illuminate. To begin, a foil is a character who possesses traits that contrast the traits of another character, often the protagonist, in order to illuminate the qualities possessed by that other character. We might consider both Adele Ratignolle as well as Mademoiselle Reisz foils to Edna, each accomplishing something different.
The contrast with Adele shows us how short Edna falls when it comes to doing her duty to her husband, Leonce, and behaving in accordance with her Creole community's mores. Adele is quite feminine, a "sensuous Madonna," and a "mother-woman," who spends her time ministering to her children and performing her wifely duties as though they were a divine calling. She keeps up her music to beautify her home and to bring joy to her husband. She would do nothing that might embarrass him or subject him to ridicule by his peers.
Mademoiselle Reisz, on the other hand, shows us how very far short Edna falls when it comes to becoming an artist and giving up the need to be accepted by society. Reisz is invited to parties in order to be the entertainment, not because she is accepted as an equal. However, Edna is unwilling to give up ties to society completely, and she still craves acceptance and praise, especially from Adele. Reisz says Edna must have strong wings to soar above the plain of tradition, but Edna is not strong enough to do so alone.
Both of these characters illuminate Edna's inability to choose a role. Initially, she chooses to be a Creole wife and mother, but she finds that unfulfilling. Then, she wishes to be an artist. She dabbles without any real, serious talent, but she finds that unfulfilling as well. She must be willing to give something up in order to have each one, but neither option is, in the end, sufficient to satisfy her.
To address your next open-ended question, the tension between Edna's outward conformity and inward questioning illuminates the theme that the individual is almost always at odds with society in some way. Further, the tension conveys the idea that the price of total freedom from society's rules is alienation from that society. Edna cannot break the rules of society and still be accepted by that society. No person can.