I'm not sure how much help this will be for your specific items, but I can show you below what I've done with eyelash yarn.
The first is a scarf that I used the eyelash as a carrier yarn along with a bulky boucle. This was made with 1 strand of Lion Brand's Fun Fur and 1 strand of Lion Boucle held together throughout. The boucle yarn gave it warmth. The eyelash seems more for show, rather than warmth. Take note, the boucle shows through alot - the eyelash yarn literally looks just like long eyelashes. The eyelash added minimal bulk - so I used precisely what the scarf pattern called for with regards to the boucle and just ensured I had the same yardage of the eyelash.

The second is a scarf that I used Lion Brand's Tiffany yarn - which is an eyelash yarn very similar to Fun Fur. In this case, I held 3 strands of Tiffany yarn together to made this scarf. It is quite warm - maybe because it's 100% nylon and holds in the body heat. And 3 strands of the eyelash yarn were used, so I'm sure that helps with warmth. In this case, the eyelash yarn looks more like (and feels like) one of those really long-haired fluffy cats as opposed to just whispy eyelashes.

So I definitely agree with the 2 strands for the hat - 1 acrylic + 1 eyelash seems about right, then eyelash for the edging - but maybe 2 strands of eyelash. As far as the scarf, guess it depends upon the look you are going for. If you want it to feel like the long-haired cat, you might need 2 or 3 strands held together. If you want the eyelash to be more like whispy eyelashes, then I'd do what you are doing for the hat (acrylic + eyelash).
Check out my website at:
http://www.naztazia.comMy how-to videos on YouTube:
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