The PCs are poetry here, so you need to make the other stuff similarly poetic. Faerie leans that way, should be easy. Nazis … are not, especially faceless Fascist thugs. I suggest making them enfaced metaphors - one for each grievous sin. One Nazi is a coward, one is a glory hound, one an unforgiving martinet, and one other a Lothario who has set his sights on Lady Alice. This makes things even more overtly mythic, but it works to strap the bicycles together. (Note I am binning the BUF guys in with the actual Nazi and Lady Alice under “Nazis” for the rest of this.)
The something like this: shortly after entering Faerie, the PCs meet the Nazis and a firefight starts … and the guns are worse than useless. No doubt steel will be drawn, but before the fight can really get going the whole batch is set upon by the Wild Hunt or similar, and if they don’t fight together they will lose (fork the adventure to a similar one in a Faerie prison camp).
Once the Hunt backs off, the PCs and the Nazis have to work together to meet their goals, with Lady Alice probably working as the best liaison. The Nazis are after the caldron and don’t care about the son, at least initially and superficially, so this is all possible with good players and a deft touch. During the following scenes try to get each PC to sympathize with their Nazi counterpart.
Then you run the series of tests. Preferably each PC gets one test and there is always an opportunity to gain assistance from their Nazi mirror, and it is always a significant help.
When they reach their goal, there is a clear opportunity for them to “win” the scenario by betraying the Nazis, and an equally clear indication that if they don’t the Nazis will win by betraying them.
If they choose betrayal, they defeat the Nazis but Aurthur refuses to return. He has seen them for who they are and they have not changed. He tells Elain he knows of his successor, gives her a drop of blood on flower or similar rubbish as a symbol of his abdication, and returns to his faerie reverie.
If they choose not to betray, the Nazis do, and the PCs are all brutally backstabbed, possibly literally, and die. They wake up in the bodies of hounds, as Arthur leads them in a chase to catch the Nazis, there is satisfying bone crunching, and Arther sits down with them, forgives them, and makes his faerie friends take care of them in their 101 years of penance since he must return to his mortal duties.
This is cheesy as Wisconsin hell, and has a serious problem in that I personally think that anyone who bargains in good faith with a Nazi is a sucker, but it works as a story.