A couple thoughts on the feet...Remember as a child you had those paper party blowers that you blew into and the paper tube unrolled and extended out beyond the mouthpiece. What if you made something similar to that except much bigger? You kept the air supply blowing into the feet/legs and when you needed them to roll back up you simply deflated them? Another idea comes from our production of A Christmas Carol, with the ghost of Christmas Yet to Come we made the finger point on a huge puppet by threading elastic through some pvc pipe holding the jointed sections of pipe taut. When the elastic was pulled the finer looked clenched but when the elastic was let go the finger shot out as if pointing. With the feet you would have to have the two feet with the taut elastic and when needed the elastic could be pulled and the feet would roll up.

A production I did years ago simply stuffed some long, striped socks with crumpled newspapers, placed the shoes on them and set them under the front porch of Dorothy's house during the "tornado" scene change. (Dorothy's house was built on a wagon with swivel casters -- stagehands actually spun the house as it travelled from Stage Right to Stage Left)

When it came time for the witch's feet to disappear -- another stagehand (offstage actor) simply reached under the house from behind and slowly pulled them off, under the porch.

I'm not sure what the sight-lines at your theater are like, but remember, the effect happens in less than 20 seconds and occurs ON THE FLOOR.

Don't waste spending a lot of time, effort and money for a quick effect that might be visually blocked from anybody sitting two rows back.