I'm learning vim bindings.
Is there a feature (or a tool) that, for a given command, will flag up any optimized/shorter way to perform it, when there is one?
For instance,
if I do (while in normal mode) i + →, I want a warning to be triggered saying that a is shorter to perform the same action.
You should understand that vim is highly customizable. It's so that is prone to do that mistakes that you mention. But, the only way to know that there is better shortcut for an operation: It's first check the vim documentation and then check your owns functions. There is no such algorithm, plugin or function in vimscript that could enable detect that as far as I'm concerned.
You should read all chapters of learnvimscriptthehardway.stevelosh.com, understanding how vim works as a result you should know how to handle your owns functions, mappings, etc.