First, with regard to luring the Down, you're likely holding the treat too far out. Move it much closer into his chest. You don't need to *move* it down so much; let him see/sniff the treat is in your hand, then put the treat near the floor right smack between his paws. He will likely 'back up' his body into the down position at first, but that's ok. He'll get it.
As for getting your dog to just lie down by the front door, well, LOL, welcome to the club! My guy is 2 yrs old, rather well trained, but gosh, the front door is the portal to All Things Exciting! What I found works best is NOT let him get right by the front door. Figure out a logical place that's about 4-6 feet or so back from the door(for us, it was having his butt up against the front of the sofa when he lay down). This way, there was a *physical* cue to him that he was in the right place. If you have an entryway, maybe you can have your pup lie down right where the entryway meets the living room carpet. Or maybe a little corner by a table, something like that.
THEN, train him to that spot. Put him on a 6' leash. Since he knows Sit, start with that. Tell him to Sit, then Wait. Then walk to the front door. If he rushes the door, tell him "uh-oh!" and move him back into the sit position. When you get about halfway to the front door, happily release him with a treat. (If you can't even get halfway, decrease your distance.) Do this until he understands that he has to Wait until he's released. Continue to do this til you can make it all the way to the door without him hopping up. (This will likely take several training sessions over *at least* several days, probably a week or so for it to really sink in).
Once you can get to door, as soon as you put your hand on doorknob, he may pop up. That's ok. Start all over. If you opened the door, close it. If he keeps popping up at this stage, go back to working a shorter distance (so that he gets success that can be rewarded. Training is only fun when you have some success!). Then try opening the door again. Honestly, you will likely close the door in your dog's face quite a lot before he realizes that the ONLY way to get the door to stay open is for him to remain seated. Just keep working back and forth -- short distances, longer distances with success, then try opening the door. Once you're actually able to open the door, hurry to release him (woo hoo! the ultimate reward!). Then, once he really understands that Wait means the door gets opened, you can work with him on building up time after the door is opened and he gets released.
Once he learns Down fully, you can easily replace the Sit with the Down.
Eventually, your goal is that whenever you head toward the door, your dog goes into his special spot. Then you can open the door, walk out into your yard, in and out, whatever, and your dog will remain seated. That requires a lot of self-control for a dog. It's something that we're constantly working on with my 2 year old. He knows his spot well, and usually immediately lies down there, waiting. Some months we make great progress, like he'll hold the Down for an extra minute or two. Then we plateau for a while. It's all a work-in-progress. Just keep working on it!
