I believe the following:
-Is it based on behavior?
Yes and no. Behavior is primarily an indicator, a symptom of the deeper issue. Behavior doesn't always reveal what's in the heart.
-Is it based on disposition?
Yes. Honesty before God concerning one's own soul. This should bring confession and humility.
-Is it based on following a set body of laws?
Yes and no. Is it based on the Mosaic Law code? No. Is it based on the law of love? Yes. So one is called to do two things: 1.) Love God with all their being. 2.) Demonstrate that love for God by loving others as themselves. Beyond these two laws... the rest is simply commentary.
-Is it based on a legally imputed righteousness in God's eyes?
Only with regards to justification and it's relationship to prevenient grace.
-Is it based on an imputed righteousness that comes from spiritual union with God?
YES! You are holy by virtue of God's own Spirit indwelling YOU. And as you learn to submit to the inner promptings of the Spirit (sanctification) you will grow in Christ-likeness. The process has a meaning and a purpose. It is to teach you about yourself... your nature... as you relate to God and His nature. The end of the process is... glorification in absolute perfection at the coming of the LORD.
-Is holiness relative to the process of sanctification?
No. Holiness is the state of being one with the Holy. One is "holy" even as behaviors change through sanctification.