We cannot confirm that an Imam has recited this supplication. Yes, it has been reported in some books that the said supplication is narrated from one of the Imams (peace be upon them). Supplicating to Almighty Allah through the Prophet or the Imams (peace be upon them) is amongst the recommended deeds, whether we use this particular supplication or another Dua.
If there is doubt in her religion, whether it is Christianity or not, then one must make sure that it is religiously acceptable to marry a woman from such a religion. If the marriage contract took place without checking the religion and it was not religiously permitted, then this act is prohibited and considered as adultery.
After choosing a certain Marja who is qualified to be followed and following him, you should continue following him, until it is proven to you via religious evidence that indicates that there is a more knowledgeable living Mujtahid than the current Mujtahid. It is not permissible to pick verdicts from another Marja just because it suits you.