I'm not nearly as experienced as JD, especially not at teaching people to use Inventor, but I agree that the instruction you have received to this point is inadequate, to put it politely. Please read through the document that JD referenced (in message 22).
In addition, many of us would like to take a crack at the problems which you claim show Inventor in a bad light-- many thousands of people are using Inventor every day to create very complex models and assemblies, invent cool new things, and generally make their living. Inventor isn't the perfect solution for every design task, but it is very usable.
The attachment is my first shot at modeling the file you attached. I started out with the first sketch constrained symmetrically about the center, which simplified most of the succeeding features. Each sketch from there on is fully constrained/dimensioned. I observed that many of your sketches had no dimensions or constraints at all-- how can you design something like that? To make any changes to the design would be incredibly difficult.
I also added a little clearance to your 12mm holes and slots, since in the real world you will have difficulty forcing a 12mm fastener through a slightly undersize 12mm hole, or finding an 11.8mm screw.
I will be away from the office and from Inventor the rest of the week, so if you have questions about my modeling technique, you may have to depend on others for answers. I hope that JD has time to coach you through some modeling tasks. Most of us here have learned more from him than from books or classes. You might search through the forum for other cases where he has tutored a new user in various techniques.