Inventor is truly not for the faint of heart. It is for engineering design and engineering drawing creation. I prefer to use it because I have been using it professionally for 20 years. I offer the design itself for beginners to build physically with their kits. As far as CAD goes, IF one wants to know Inventor, (or Fusion, SolidWorks, SolidCAD, CATIA, etc.) so they can design what ever they want from scratch, I recommend looking into a training path to do so. If you are looking to just assemble VEX IQ, SNAPCAD would likely be an easier option.
To answer your question about designing this in Inventor, I downloaded the STEP files from the VEX website and assembled it. I tried a few different combinations and iterations until I got what I felt would work. After I has purchased my first VEX IQ kit, I then built my design and tested it. I then refined it. There really is no substitute for a physical prototype. It is the only way to find and work the 'kinks out."
The next one I plan to post I call the “Klann Krab” and it utilizes the Klann mechanism. I work for a local college teaching CAD and Machine Design to students. In contrast this particular design was built first with my VEX IQ kit. The CAD will be used to document the build so I can build it again next year.
Video of the Klann Krab.