What should I do about my Christian wife who confessed to adultery and wishes to return to God and embrace Islam after I had intended to divorce her?

If the adulterous wife repents and mends her ways, there is no harm in keeping her as your wife. You should seize this opportunity to encourage her in Islam. An adulteress, whether a wife or not, must perform istibra' (cleansing of the womb) according to the more preponderant opinion. Scholars differed on whether istibra' is with one menstrual cycle or three, but the former, one menstrual cycle, is more preferable.