Homebrewing - Beer, Mead, Wine, Cider

my father ferments lemon peel all the time so i guess that's not it. is your peel organic, or at least well cleaned? i imagine that the chemicals which prevent the lemon from spoiling are the same which impede fermentation

Why do you think it's stalled? Maybe just lemon peel trapped the gas?

I've made lots of mead with citrus peel, including lemon, I've used in on secondary though, just to keep more flavor by less gas escape. Maybe you should do the same?

I used to work at a pro brewery that made a beer with loads and loads of lemon peel and juice (no bitter pith, thank you) and it absolutely does not kill fermentation.
My fingers and thumbs still hurt from the early days when it was quite a manual job!
That said, you could probably add so much juice that the acidity might kill fermentation.

One more brewer here routinely making a lemon and ginger infused beer and no problems there. I typically have four large lemons sliced thin for 20 litres of wort. I infuse the lemons and ginger separately and add the infusion late in the boil. I always wash the lemons with hot water to get rid of anything they might spray them with.

Kefir might be different though (I don't know anything about that process). But maybe making an infusion vs. chucking the peel in the fermentation might help.

If lemon peel was this reliable at halting fermentation, you'd expect to see a lot more traditional recipes that have a hint of lemon.

You're sure you've got all the info you need to call it? This kind of thing is hard to assess without gravity readings.

I've done mead with lemon zest before; it fermented as expected. That ingredient by itself should not cause fermentation to stall.

Definitely not. I sliced 50 lemons up and fermented just those (everything included) with 12 pounds of sugar or so and the resulting wine was 9%. I didn’t even bother balancing the ph or anything either.