Maybe it’s the Australian coffee snob in me, but I think the better way to solve this problem is not to change the mug, but the coffee.
Drink proper, espresso coffee, using milk steamed to no more than 65ºC. Your mug’s only too hot because your coffee’s too hot.
You get to drink more delicious coffee, and if you still want to pursue the coffee sector, there’s probably more money in distributing and marketing espresso. People will pay for it every day, rather than pay for a mug once.
Don’t let me be seen to be hating on your idea here, you’ve obviously put a lot of thought into it and the design is beautiful, this is just a perspective from a place (Australia) where we don’t experience the problem you’re trying to solve.