mistakes can be corrected any time however, lies, fraud, not enough evidence, no evidence at all and people who don't qualify but applied anyways will get denied that for sure. you can relax if you went over everything with your lawyer. they don't really care for cover letters unless you are going to immigration court which you are not. the cover letter here is for fomality. the immigration agent will have the cover letter to guide him/her through all your evidence hopefully or will just have to find the evidence for their self. they are trained for disorganized packages arriving.