Did Ibn Ezra/Rashbam/Radak/other pashtanim believe what they wrote?

The footnote at the end of letter 31 - part 2 in vol 4 of Michtav has become well known these days because it deals with the issue of apparent contradictions between Chazal and  science, a topic widely discussed mei'Hodu v'ad Kush in the Jewish blogsphere.  I want to call your attention to the body of the letter, which raises a no less important issue, albeit less well known.   Rav Dessler in that letter distinguishes between two types of derashos of Chazal: 1) derash which is obviously not meant as a literal interpretation of the text, but rather adds a level of meaning above and beyond what the text says; 2) derash which serves to illuminate the plain meaning of the text by filling in lacuna, providing context, explaining obscure words, etc.  This second type of derash fills the same function as what, if it came from another source, we would call pshat.  Which brings us to the key question: if Chazal give us "pshat" in pesukim, asks Rav Dessler, why did the Rishonim nonetheless still engage in learning "pshat" in those very same pesukim, oftentimes in ways that contradict or ignore the interpretations of Chazal?  How can the Rashbam, the Ibn Ezra, and others push aside Chazal in favor of their own reasoning?

I find Rav Dessler's answer striking.  He suggests that these Rishonim only wrote for the "nevochim," those poor confused souls who could not accept Chazal's interpretation of the text, but were willing to accept the text as true given some other more reasonable (in their eyes) interpretation.  The Rishonim wrote to demonstrate that 1) the text does lend itself to multiple interpretations, apart from the one given by Chazal, and 2) there is no inherent problem in accepting those other interpretations as pshat provided they do not contradict any fundamental theological principles.  Therefore, even if you have trouble digesting a pshat of Chazal, your spiritual goose is not cooked, so to speak.  By way of analogy, Rav Dessler compares the efforts of the pashtanim to the Rambam's Moreh -- a book intended to provide guidence to a specific audience with specific needs, not a book meant to provide the ideal of best answers to all philosophical problems.  So too, the Ibn Ezra, the Rashbam, the Radak, meant to address the needs of a specific audience, not to provide the "ideal" interpretation -- that is limited to the words of Chazal.

In other words: "pshat" is a b'dieved, a crutch for those not yet comfortable or not yet ready to accept the "gospel" (excuse my terminology) of Chazal's explication.

And so I ask the question which I used as the title for this post: did the Ibn Ezra believe what he wrote?  Did he think his interpretation -- where he differed from Chazal -- was correct?  Did the Rashbam?  Surely these giants were not "nevochim" themselves.  So when we read an Ibn Ezra that tosses aside a Midrash and suggests some other reading of a text, are we to think that in his heart the Ibn Ezra really believed the Midrash's interpretation to be the most plausible, and his critique is just a ruse, i.e. he is just going through the motions of presenting the text in a way that he thinks a "navoch" would be comfortable with? 

(I don't know if there is an online Michtav I can link to, otherwise I would.  If someone finds one, pls let me know.)