Is it permissible to perform Tayammum for major ritual impurity (janabah) if it is impossible to perform ghusl with water due to its extreme heat?

What you mentioned is not an excuse that permits tayammum, as the heat of the water can be removed by collecting it in vessels or by purchasing non-hot water, which is abundantly available. Therefore, your resorting to tayammum is not sufficient. The cure for obsessive whispers (waswasah) is to ignore them. Ghusl is achieved by making water cover the entire body, and a strong presumption is sufficient for it.