The most important thing is to have a great manuscript.
If the agent/publisher says they want to see 5 pages, you really don't have 5 pages. You have 2. If they are bored after 1-2, they won't get to the fifth page.
Marketing trends. Remember that when you send the manuscript, it's a good 4 years before it comes out. So if you decide to write something that is 'hot' now, it will be old news by the time it comes out.
It is possible to make as much in the LDS market as it is in the national market. You can make more in the national market, but it is very possibly to do well in the LDS market. However, you will probably make less in the LDS market.
One of the downsides to LDS market is if 2 publishers (really just one) turn you down, you don't have many other places to turn. In the national market, there are dozens of publishing houses.
Self publishing just does not work. There are too many new books coming from publishers every year to leave shelf space for vanity press books. The only reason you would ever self-publish is if you want a few copies to give to family and friends, or if you have a massive outlet to sell your book (example, motivational speaker to thousands, and can sell your book after speeches).