Dwell clicking software that supports right-dragging and allows to define shortcuts
DwellClick is a nice dwell clicking app on mac that is customizable, versatile, as well as nice-looking. The following pictures show the hands-free panel and the pop-up panel of the app.




It supports most of the features you mentioned:


single/double click, drag, drag &amp; hold
keyboard shortcuts for changing modes
hands-free panel that stays on top
customizable dwell clicking time


In addition, there are other features such as:


modified click with ⇧, ⌃, ⌥ or ⌘ 
pop-up panel near the mouse pointer to select different modes


The only modes missing from your inquiry are triple click and right drag, but I don't think Mac OS uses these modes. However, it should be easy to configure a desired short cut or action to the modified clicks using BetterTouchTool mentioned above, so that you can use them instead.