The door, as said before is the connection between teh two worlds: Hagaren (Where Ed and Al is from) and Circa (where Hohenheim was sent to from Dante through Rose's baby). As I see it the door can only be opened if somebody from Hagaren died or failed a Human Transmutation because thats where they arrive in front of to see the so called Truth of alchemy by the creature that is kinda like the guardian of the door.

The Homuculous are failed humantransmutations. They can have many lives due to the "red stones" that are given to them to eat...once those runs out in their body they die. And yes they are weakened...or rather immobile from seeing or being touched from an item of their previous body or life. Combiined w/ that and a seal you defeat a homuculous.

As for the ending of FMA...the very ending just well...symbolizes that Ed and Al will find a way to overcome the gate and find each other again

@___@ i'm a bad explaniner, hope this helped