Mission accomplished!
Thanks to all who helped me, in many different ways, over the past 7 years. It's been quite the adventure, physically, mentally and spiritually. A lot has changed in since I was paralysed by panic on the summit of Mt Anne, convinced a safe descent was utterly impossible for me.
In the end, I couldn't have summited Fedder without a hoik up on 2 or 3 occasions. The scary part was, as I expected, the bulgy rock on the ledge near the money shot:

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Sorry I can't reference the photo - I got it from the internet some years ago (the person normally has a face), and it's been up on my wall since then.
Part of the bulgy ledge was easy, with lovely finger holds. Part of it wasn't, where I only had splayed hands contacting the rock above me. The combination of that and having to lean my middle out over nothing was definitely adrenalin-inducing. I focussed as much as I could on what I was doing, but I probably rushed a little just to get past it. Taller people might have nice finger holds, and some apparently don't notice this bit.
Though it was challenging, I didn't find any of Moss Ridge scary, even in the rain. Geeves and Chockstone Gullies were fine for me too. They just required a lot of care, especially to not dislodge any rocks onto the head of anyone below.
Sliding about 3 m down the wet rock near the base of the direct ascent was an interesting experience. Instead of confirming my fears, I just felt peace, and made sure I didn't risk any more holds that I wasn't as close to 100% sure of as humanly possible. That's where the careful hoiks up provided the crucial, solid 3rd point of contact. I had slipped off a wet, slopey toe hold as I pushed off it. Fortunately there was a flat bit below me, and another member of our party there who made sure I stopped.
The walk coordinator had been up Fedder several times before, including on wet rock. He had found that up higher, where it was more vertical, it was actually less slippery. Maybe there's less lichen on the more exposed rock.
I'm glad that I later got to a point where I stopped and said I'd have to leave it there, because the next scramble was too dangerous for me. But with a good look around, and another careful hoik, it was no longer dangerous. (Thanks!!) It was good to know I could still make wise decisions, and not be consumed by summit fever. But even better to find that there was a way after all, with the right help.
One day I'll write up the story, but meantime here are a few photos: