Is water that has mixed with urine on the thigh and then dried considered impure or pure? And what is the ruling on doubting whether impure water has entered under a skin peel on the foot, knowing that its opening faces downwards?

If you have a doubt that urine might have touched your thigh, the default assumption is that it did not, because impurity is not transferred merely by doubt. If you are certain that it has touched it, then it is sufficient to pour water over the impure area to purify it. The water that separates after removing the impurity is pure as long as it has not changed [in color, smell, or taste] due to it. However, if you doubt whether the water used to remove the impurity reached a specific spot, the default assumption is that it did not reach it, but even if it did, it is pure. You must ignore obsessive whispers (waswas) and avoid giving in to them, as they lead to great evil.