Hey Geebs,
I think we're talking about two different sources of feather sexing. The most common is with the k+ fast feathering gene in Mediterranean varieties crossed with the K slow feathering gene in Asian varieties, e.g. your Cochins.
Both K and k+ exist in most varieties of chickens, and it works like barring. Your hen has to have the dominant form: K or slow feathering for it to work for sex-linked gender determination.
Probably within certain Cochin lines the k+ gene has been eliminated or maybe even never entered the picture if the lines are pure. In the case of these cochins only, the presence of two slow feathering genes in the roosters KK would cause them to feather more slowly than the hens that can only carry only one K. That is why you would have slow feathering roosters and hens that feather faster. I believe that it's really just that you have all slow feathering offspring, and the roos have a double dose of slow feathering and are distinguishable from hens as a result. Does that make sense?
I hatched 3 wheaten marans recently that displayed textbook feathering configurations that were obvious at day 3. One hen was k+ and feathered out super fast. At 3 weeks she already had head feathers coming in. One hen was K and she's still in with my newly hatched because her feathers are coming in really slow. I have a rooster that is somewhere in between the two, so he must be Kk.
Somewhere along the way, Marans (at least here in Europe) have picked up the fast feathering gene. I can't remember offhand which of the ancestral breeds would be fast feathering, maybe the OEG? So I don't think it will ever be a consistent way to determine gender unless the fast feathering gene is eliminated from marans, which isn't likely since we have so many other things to select for.
On the incubator temperature and gender of chicks - I know that after fertilization with reptiles that temp influences gender. That's an interesting theory about whether gender could be determined before hatch by egg temp. They generate their own heat after like day 14 or something don't they? They'd have to be purely experimental eggs I didn't care about - I'd be too paranoid to let them cool to their ambient temp to thermograph them if they were eggs I cared about.