I think the issue with Drain Intelligence restoring all your mana was changed in an unofficial patch, but in official versions of the game, it seems that if you order a Fortify Magicka effect after the Drain effect, they do a strange multiplicative thing to your blue bar.

Drain Intelligence so that it goes down to 1, for 1 second. Remember to factor in magic resistance/vulnerability.

Then below it, put the highest Fortify Magicka effect you can afford, for 2 seconds so that it outlasts the Drain effect.

A spell with

Drain Intelligence 99 points for 1 second
Fortify Magicka 100 points for 2 seconds

...when cast, set my blue bar to 101/1 points. This becomes a working ratio or something which the game uses. When Drain expired, it became 10,076/100. I think it would have been 10,100, but apparently it factored in the cost of the spell, 24 points. When Fortify Magicka 100 expired 1 second after that, it only deducted 100 mana, not ten thousand. So I ended up with 9,976 mana, permanently or until spent. Cute.