Danish rye bread (rugbrød) gone wrong?
I definitely think buttermilk is required. If you don't want to invest in a whole jug, you might be able to find some plain kefir instead, or even yogurt will work in a pinch.
Sourdough starters (like the preferment like here) contain a combination of yeast and bacteria. It will definitely have a sour and somewhat funky smell, but generally speaking we've evolved to recognize good funk smells from &quot;I'm not going to eat that&quot; smells. It's not perfect, but it's helpful.
Yeast and bacteria are naturally present.... Everywhere basically. We get yeast infections on our skin and in embarrassing places because they are literally everywhere. Ever had dandruff? That's a yeast thing too. But there are lots of varieties of yeast. Beer and wine yeasts have been cultivated to produce/tolerate different amounts of alcohol for example. So even though yeast is everywhere, you added bread yeast to your preferment because you wanted that one. you didn't want some weirdo yeast chilling under your fingernails or something.
The thing with the milk and lemon juice substitute for buttermilk is that it only mimics the acidity and sweet-sour flavor for baking. But the bacterial profile is not the same. It could be anything! Some bacteria naturally present in milk produce iodine, which is bitter. For all I love my ferments, I can't predict whether my milk will become a delicious mesophilic buttermilk or a jug of bitter sadness if I let it spoil naturally..... But that's kind of what you did here.
Your milk and lemon juice both contained bacteria in small amounts to start, and nobody knows what they were. After several days, those bacteria multiplied by a lot, and your nose told you that whatever they were, they did NOT make something delicious. I think you were right to toss it.
So, while you might not have to use buttermilk, I would definitely use some kind of cultured dairy. Buttermilk or kefir would be best because those are room temperature mesophilic ferments. Yogurt will work in a pinch, but they're typically predominantly thermophilic cultures, meaning they flourish in a warmer environment.