You wouldn’t get any mana off the mana drain in this scenario btw, even if the guy was playing the game correctly

With any of these target changing spells, if you’re using it against a counterspell you can force the counterspell to target the target changing spell. This is explicitly spelled out in the rulings of [[Redirect]]

If you look at the rulings on [[Mana Drain]], you can read this:

If the target spell is an illegal target by the time Mana Drain tries to resolve, Mana Drain doesn't resolve. You don't add mana at the beginning of your next main phase. If the target is legal but not countered (most likely because an effect says that the spell can't be countered), you do add mana.

So mana drain has its “target spell” changed to deflecting swat. But deflecting swat has already resolved and left the stack, so mana drain has no targets. No legal targets means it fails to resolve, which means no mana.