I can tell you it's so true that it's really hard to figure out what's going on. Everybody will tell you different things, different banks and even friends and family.
I couldn't stand it, so I sat down and researched the mortgage calculator formula. It's actually pretty easy:
http://www.hughchou.org/calc/formula.html
I implemented this in javascript and was able to see my monthly payments and project my loan amounts over the years. This gave me a great way to fiddle with lots of parameters and instantly see differences. I was able to graph the results and see exactly where I broke even on different investments, etc, and completely decide, without persuasion, what kind of loan I wanted.
Most loan calculators suck. They don't really let you play around with the parameters, or they aren't clear on what they are including.
I'm happy to see this company because I'm all for opening up this kind of "fiddling" to people who can't program. I feel bad for people who are confused but don't have the skills to sit down and really figure it out. I wish you luck and I hope you better people lives with this.
EDIT: I played around with your tool a little bit. Here's where I think you guys should go: financial advice. Don't force people to tell you what down payment they need. If they don't enter one, suggest a few different amounts and show the difference it makes. If they don't specify a 15 or 30 year loan, show them the differences (pros and cons) and let them decide. In some cases, persuade people to make good financial decisions. You should rarely get a loan without any down payment. Tell people that.