The song is in New High German language, a dialogue between two lovers. The forest bird, could be a messenger of love, as so often in German folksong, but in the last verse it is revealed we have a simile: the bird is the lover. Maybe he is living in the woods (outlaw?) or a knight errant - there are many possibilities.
The speakers: 1. male, 2. female, 3. male.

Some of your grammatical questions:
leeneia - you must read your dictionary with more care. Schleuse is a sluice, but schleuss is an old infinitive of schliessen (to close, to lock). Here you guessed right: it is to enclose.

"Vor" and "in" are prepositions reigning either the dative or accusative. When used to describe a position (rest), they reign the dative. When used to describe a direction (movement), they reign the accusative. With "in" you have it in English, too. Position: Ich bin im Haus = I'm in the house; movement: ich gehe ins Haus = I'm going into the house.

The snow white hands don't belong to the bird, but to the female adressed by it.

Songs with a dialogue must not have their origins in plays; there are a lot of them going back to times of old (Tragemund's song, or the Blue Stork: song, thread.

Paul - A Marian song in code? I dont think so, it is a love song.