Hi, Elena
I recommend using chicken thighs, but keep the skin on.The method is similar to that of scallion chicken. You can refer to this method.

haha,西北大学 I know it，I graduated from Xi’an Jiaotong University and 孜然牛肉夹馍 you mentioned are indeed very delicious.

You most certainly can freeze. I’ve been doing so for 30 years with no ill effects.

My go to jiaomo are 孜然牛肉夹馍 (cumin beef jiaomo) which I was taught to do by a 70 year old chef of a restaurant very near 西北大学. He regularly froze the dough.

Hi，Connie
Adding milk to the dough can give you a better taste. You can choose to add some appropriately.

Sure, beef brisket.

Marco,
Each spice has its own flavors and aroma. So there is no really good substitute. You can simply skip the two.