You can have mapped keys execute a step on Press, Release (Default), or Press and Release.

The IWT command, is "interact with target", as soon as warcraft receives this, your toon begins to interact with target... which means (if Click to Move is on), your toon will begin to move.
Even with Press and Release, the press triggers the IWT which begins the movement.

I'm not sure if this will work as intended or not.
As in, will an IWT interrupt spell casting when in melee range with the target.

I'm fairly sure the IWT will (when CTM is on) turn your toon towards the target.
And if you're already in melee range, will not initiate movement.
Turning alone, does not interrupt spell casting.
But I'm unsure if IWT will turn on melee combat (which is the default interaction, with a hostile) and interrupt casting, or if the casting will continue and melee initiate once the spell is done.