I'm afraid I've never heard of Manfred Pienemann, nor of Malcolm Johnson. Perhaps that's because I'm not in Oz (although I've done some reading of some of the Australian thinkers in this area of language acquisition--Christopher Candlin springs to mind). I have, however, heard something about the ideas you've expressed here regarding order of learning. Although I gather that research in this area is barely begun, it seems reasonable to me that while there may be some kind of overall organization to learning order as you've suggested, the notion that particular learners will deviate from the normal pathway is hardly surprising. If true, one wonders then, of what real value would research in this area be if all we could say was that learners often follow this path, but many do not.
It all seems to support the reality we all experience, as teachers, that whatever we "cover" in our classes, many particular students will not pick it up. What is universally required for all students is to revisit what they have been exposed to before--often many times over. We simply must not conclude that because we had a unit on present simple tense at the beginning of the term, our students therefore, now, near the term's end, know present simple tense--or even that they should. A language is an organic whole. It is artificial to break it down into many "parts" to be studied individually, then expect students to be able to put all these "parts" together again in some sort of synthesis of the language. In a very real sense, one cannot understand present simple tense until one first understands all the other tenses and how they differ from present simple. So, we and our students are compelled to go round and round in pursuit of mastery of our subject. In that process, particular students will learn parts of it in different orders than others. But there seems to be no way to avoid the circular studying and learning process. Sadly, our methods do not reflect this apparent reality. We insist on labelling students with 'level's and on using materials which progress in rather linear fashion from "beginning" to "advanced".
Am I misrepresenting the situation here, do you think?
Larry Latham
Just to put my two cents worth. This is my first post in this forum. I agree with you all that it depends on the kind of class you have, and what the aim of the students who are learning the language is. For me it is important to talk about grammar with my students not so much to learn the names of components as to understanding and being able to use the functions the names represent. My approach to teaching is like that of any instructor, first I show them the waters, then I have them jump in. Feel what floating feels like, then learn to swim. Later we can worry about style. I find that the beginning of the class is a great time for discussions. I like to introduce the grammar points to be presented in that day's class in a conversational way. Later, in the class we work with time-lines as visual aids for example, in the case of tenses. This has helped me help them. 
Be well, be safe. Bob