Maybe Diriel could be captured while out searching for ore or other raw materials.One disadvantage of talking about the season long story arcs up front is that it's been long enough that we've all forgotten what we talked about
Círdan aiding Fingon is likely meant to be in the next episode and is just 'left over' from when this was the Dagor Aglareb episode. Meaning, I think it's assisting with the attack down the coast. At the very least, it's showing that Círdan does not let the Ban prevent him from working together with the Noldor.
But I think it's fine to keep this an Angband-centric episode, leave the audience in suspense (at least a bit) as to what's going on with Fingolfin and his reaction to Thingol's Ban.
Certainly, the focus of this episode should be on what happens to Edhellos, so having Curufin's wife be the eyes through which we see that works.
Edhellos' story: Prisoner in Angband. Refuses to aid Sauron, who is frustrated because he thought he'd broken her. Morgoth, who has returned from the East to check in on Angband(?) essentially says 'watch this' and puts the SoBD on Edhellos. Sauron is majorly impressed. Edhellos is broken - completely. She's crushed. She perhaps can't even speak coherently. Morgoth can use her as a puppet. After demonstrating this, they abandon her, leaving her for the Noldor to find. The Noldor find her, and while dismayed, try to treat her.
Díriel's story: Captured (how?) and brought to Angband. Meets the captive Edhellos. Sauron tries to get the elven prisoners to work for him, with very limited effect. Díriel refuses. Morgoth works the Spell of Bottomless Dread; the elven captives see the aftermath. Suddenly, cooperating with Sauron becomes a much more reasonable option. Díriel (and others) become slave labor in Angband, aiding the war machine that is fighting their own people.