Does removing clothes and touching one's wife with the intention of intercourse, without it occurring, nullify ablution and necessitate ritual purification?

Merely undressing does not invalidate ablution, unless desire is stirred and pre-seminal fluid (madhy) or semen (manyy) is discharged. If what is discharged is semen, then ghusl (ritual bath) becomes obligatory. If it is pre-seminal fluid, then only washing the male organ and performing ablution are required. As for touching a woman without any discharge, and touching the male organ, there is a difference of opinion among scholars regarding these actions.