"Will", "Going To Do", and "Doing" - why learners get them wrong

Here's a little example of something English learners get wrong that I attribute to a failure of textbooks and teachers to properly reflect the way language is used.

I've been spending a lot of time recently hanging around Lang-8.com and helping people out by correcting their journal entries. Most of the corrections are novel - they're things that I've never encountered before.  Others are so common that I barely even think of them - problems with plurals and articles being the main example of this.

There are a few types of mistakes that are common enough that I've developed a standard response to them but not so common as to seem hopeless.  One of these is the over-use of "will".  Here's a made-up (but not too far from what I've seen) example:

Tomorrow I will go shopping with my mother.

Some might argue that this is a grammatically correct sentence, but in my opinion, it's unnatural and not something I would say unless I was doing it in a funny robot voice.  The better phrasing is:

I'm going shopping with my mother tomorrow.

When you tell an English learner, the inevitable question is, "What is the difference between "I'm going shopping," "I'm going to go shopping," and "I will go shopping"?  This is where things get murky and I start replaying real and imagined conversations in my head to see what works.  A grossly oversimplified answer:

• "I'm __ing" is for things that you have solid plans for: "I'm meeting them for lunch in 10 minutes."
• "I'm going to" is for things you intend to do: "I'm going to wake up at 7:15 tomorrow."
• "I will" involves deciding (an act of will, if you will...) at that moment: "OK, I'll call you later then."

I like to lay blame on people, so my question is, whose fault is it that these language learners are over-using "will"?  I blame it on the grammar textbooks. By presenting it as the "future tense" and making it a thing to be conjugated and manipulated rather than understood for meaning in a certain context, traditional grammar-based lessons put the wrong ideas in those impressionable English learners' heads. I bet you can probably guess what I propose as the solution