Thank you for this question! Let me offer a few points that can help with this question.

a) You wrote that a Bas Yisrael who is Mekudeshes to a Kohen is prohibited mid'Rabanan from eating Terumah because she might give the Terumah to her parents and family. The Gemara in Kesuvos 57b brings this reason in the name of Ula. But then the Gemara brings another reason for this Gezerah in the name of Rav Shmuel b'Rebbi Yehudah. We do not allow her to eat Terumah because we are afraid that after the Kohen actually marries this woman, he might find a Simpon -- a major disability or physical blemish which he was not aware of -- and the Kidushin is considered a Mekach Ta'us and nullified retroactively. If this occurs, it turns out that she was never allowed to eat Terumah in the first place. It seems that the Gemara prefers this reason l'Halachah. If so, in this case, especially according to Rebbi Eliezer who says that the marriage is meaningless, this reason does not apply.

b) We may suggest also that even according to Ula's reason, there might be a difference between an engaged woman and the case of a Ketanah. There was a custom to send gifts and fruits to the engaged bride (Sivlonos; see Bava Basra 146-7) which is the basis of the assumption that she might offer tastes from these gifts to her family.

c) As a rule, making a Gezerah d'Rabanan on a Ketanah is not so trivial, as we cannot really enforce the Gezerah. If this Ketanah is permitted to eat Terumah and is given Terumah in a permitted way Min ha'Torah, we cannot really prevent her from eating it. We saw above (90a) that even in a case of a marriage of a Bas Yisrael Ketanah to a Kohen which is only valid mid'Rabanan, we will not interfere even if she eats Terumah mid'Oraisa which is forbidden in any case. (See Tosfos 90a, DH v'Ocheles.)

Best regards,

Aharon Steiner