Scene elements can not be drawn on top of UMG 2D UI. You’d need to make a version of the coin you spawn in the UI as a flipbook material or a static texture widget that flies into the coin box/pouch/field.
Not sure I understand the question about drawing umg for least z-order. The order of widgets is logically determined based on the parent structure of widgets. A child widget can not draw before a parent. Some (just canvas) widgets, allow you to provide a Z-Order on children that overrides the order when sorting the draw order of the children of the canvas. However, that order will not affect the order of the widget outside of the canvas.