It's common courtesy to remove your shoes when you enter someone's home (I always did when I was a missionary, anyway), but of course people don't always remember. If you'd like them to do so, simply ask them when they come to the door. They won't mind...but in all honesty, these are teenage boys. Unless they had very good mothers, they probably won't remember to do so. You'd better ask.
"Elder" is their title--it's an office in the priesthood and leadership of our church. I know the contrast between what people think it should mean and their actual ages tends to throw people off, but yes, that's what you should call them (it also means you don't have to remember their names).
If you want to feed them, go ahead. Again, they're teenage boys--they won't refuse food and will probably love you for it. They have to cook their own meals now, and I think it'd be safe to bet that they're living on instant noodles at the moment. Still, if you don't want to do so, then just offer them a glass of water. You don't need to do more than that to meet the demands of courtesy (though I'm impressed that you care enough to ask. Most don't).
And don't worry about the discussion. They just want to tell you a little more about what sets our faith apart from other Christian denominations, what we have to offer that others don't. Hear them out, ask questions, and if you're interested in more, tell them so. If you're not...well, you've learned a little more about those around you, and how can that be a bad thing? I sometimes wish people of other faiths would send people around like this, just so I can learn. I think it makes us all more compassionate and open.