I have seen young girls with more than school, family & friends suddenly deciding that they fell in love and want to get married. The consular officers have seen it too often that it will be an uphill battle based on her current situation.
There is nothing you can do to change the outcome of a visa interview, having a romantic interest with your son and having so much support from you and the pastor etc will do more harm than good based on my experience.
If she is completely involved with the church and doing voluntary work there are avenues and visa types that the church can sponsor, this is a more formal and costly process but it is also subject to the same rules of establishing not immigrant intent at the interview.