Responsive hotspot / button on image

The main reason for your problem is that a normal Fluid Box doesn't allow stacking of objects on top of each other. That is the reason for that hotspot link which you posted, and for banning click boxes. Only in a static fluid box will it be possible to stack objects, but I suppose you did read that in the hotspot link.

There are a couple of possible workarounds if you are indeed persuaded to need a responsive project:

1. Switch to Breakpoint Views workflow. I know that Adobe is not even mentioning it anywhere, but it is still possible to convert a responsive project with Fluid boxes to a Breakpoint Views project using the Project menu. The control is much better but it may take more time to have a working project. This older blog post explains some of the differences;
   http://blog.lilybiri.com/fluid-boxes-or-breakpoint-views
   
   
2. If you are on the most recent version of CP2019, which is 11.5.5.553 (Windows), you may ignore that it is posssible to use a slide without fluid boxes in a Fluid Boxes responsive project. In that case you will set up the size/location of objects using the same Position Properties panel which you also have in a Breakpoint Views project. That may solve the issue as well. I have described this for another use case, interactivve video, which is also impossible in a normal Fluid Box. 
   http://blog.lilybiri.com/fluid-boxes-and-interactive-video-position-properties-panel

If the buttons on the ultrasound machine are in a fixed layout, why did you decide to build this course module using responsive design instead of unresponsive?  Is it really likely that many of the learners will be using mobile phones to do this course?