Is the prayer valid when performing ablution and wiping over the leather socks according to the Hanbali school, with the presence of an excusable impurity according to the Hanafi school, and is it permissible to mix between the madhhabs in this manner?

What the questioner asked about is not a conflation of madhhabs. The Hanafis permit wiping over leather socks (khuffayn) and thick socks (jawrab), and they rule that a small amount of impurity is excused (that which is the size of a dirham or less). Therefore, your prayer is valid if you wiped over the leather socks or thick socks and prayed, and the impurity on your clothes was the size of a dirham or less. However, if the socks were thin, the prayer is not valid according to the Hanafis and Hanbalis.