When you have used most of the extra mana from the INT weapon, do a Fire Blast and switch back to your usual weapon during Fire Blast GCD (pre-2.4.3 you can skip the Fire Blast). In effect you get your scorching and a Fire Blast mostly supported by "free" mana.
Except you don't keep your mana at the same level when your int changes, you keep the same % of mana.
For Example: You equip a staff with 100 more int than your normal one and gain 1500 mana for a total of 11500. You then spend the 1500 gained from the staff for a new total of 10000/11500, or ~87% of your total. If you re-equip your normal staff you retain 87% of your total mana for a new total of ~8700/10000. When you gain int, you keep the same amount of mana as you had previously, when you lose int (as you describe) you retain the same % of mana, IIRC.