You incur a risk anytime you make a significant change (e.g., changing employers midstream) that affects your lawful status. The most important thing for you to remember is this: If you get a green card, you are getting a green card only because your services are essential to a U.S. employer. Put another way, the U.S. government is issuing a green card not for your benefit, but to help a U.S. company or Institution (here a church).
Moreover, as a condition of your employment (i.e., approved I-360) your current church-employer had to provide numerous assurances. For example, that it was a bona fide nonprofit religious organization in the U.S., exempt from taxation under section 501(c)(3) of the Internal Revenue Code of 1986, and (probably) that it would ensure that you do not become a public charge.
Thus, if you change employers, you're inviting the Government to challenge the sincerity of your original I-360 application. Moreover, the new church will have to provide all of the same assurances.
THEREFORE, I suggest that you not change employers until after you receive your green card.
As to your 2nd question, as part of the I-485 process, please be aware that the Immigration Service will visit your church-employer.
As to your 3rd question, I will need more information about your case to give you an idea of what to expect next.