Hi @jmills60 John, Not quite sure what your asking here.
Question 1. Are you asking is it normal to lift the first finger off the string before applying the hammer on ? If that is what you’re saying my answer would be NO. You should keep the fretting finger on the string whilst you hammer on. The goal here is to HEAR the note change.
@BanjoBen has a unique way of playing hammer ons using his middle finger to fret the string and his ring finger to hammer on. Most banjo players fret with the pointer finger and hammer with the middle finger which is generally the more natural way to do it which may go some way to answer question 2. The duration of the note has no impact on which fingers you use to hammer with… My guess is Ben would say use whichever feels right for you.
It may help to post a short video and let @BanjoBen and others to see your progress / timing and be able to offer advice on any problems you may be having.