how can Ramban argue on Chazal - what is the "true" meaning of the text?

I wanted to write a fuller post about this issue, but haven't had time, so I figure better a chatzi shiur than nothing at all.

The Ramban at the end of VaYigash (47:18) tries to reconcile Joseph's interpretation of Pharoah's dream, which called for 7 years of famine, with the events at the end of the parsha, which seem to indicate that the famine ended after only two years (as is Rashi's position quoting Chazal).  If the famine indeed ended, wouldn't that call into question Yosef's prediction and advice?  Ramban offers three possible solutions:

1) The famine ended completely in Egypt, bas Chazal teach, but it continued in Canaan and the surrounding areas;

2) The view of the Tosefta: the famine temporarily ended in Egypt after 2 years until the death of Ya'akov, after which it resumed for another 5 years;

3) Ramban's interpretation al pi peshuto: the famine ran for seven consecutive years as Yosef had predicted, and the events in the parsha are speaking about the final two years of famine.

R' Friedlander in Sifsei Chaim (Pirkei Emunah u'Bechira vol 2 . 261) asks the following question: the famine lasted either 2 years or 7 tears -- it couldn't have been both.  The two views are mutually exclusive.  Ramban and Chazal are arguing on a matter of metzi'us, a matter of historical fact.  If Chazal, as Rashi quotes, and as the Tosefta teaches, tell us that the historical fact is that the famine lasted only 2 years, how can the Ramban contradict Chazal and tell us that it lasted 7 years?   

In other words, when the pshat and the derash contradict as to what happened -- matters of fact -- either one or the other is true.  How  can we disregard what Chazal teach us is the TRUTH in favor of some other interpretation?   If Torah she'ba'al peh gives us the facts, what license do we have to argue?

I'm surprised that R' Friedlander makes no mention of the fact that his rebbe, Rav Dessler, addresses this very same question in a letter printed in Michtav M'Eliyahu vol 4 letter 31 (post on it here). 

Be that as it may, his answer in a nutshell is that we have to distinguish between the way the Torah expresses itself, what I would call the signon, and the true meaning of what is being said.  The TRUE meaning is of course what Chazal tell us the text means.  However,  the Torah deliberately expresses itself in a way that suggests other meanings because those other meanings have value for us as well.  For example, when we are told to put tefillin "bein einecha," the TRUE meaning of those words is not between your eyes, as that is not the proper place for tefillin.  But we need to appreciate that the pshat does mean between your eyes because that also teaches us a lesson -- the ideas contained in tefillin should be before our eyes, i.e. in our minds, always.

Two points: 1) It works well for the tefillin example, but I don't see how this approach resolves the Ramban; 2) More importantly, I hesitate to say it, but I don't really think the  pashtanim understood things this way.  My impression (e.g. see hakdamah to Ohr haChaim) is that the pashtanim understood that they did have license to argue with Chazal as to what the meaning of the text is.  They understood that Chazal are suggesting a possible meaning to the text, but not THE meaning, at least as it applies outside the realm of halacha. 

Developing this idea would take a lot longer to do than I have time for now, so that's it for the chatzi shiur.  A mareh makom to the idea to think about.