Jorge, it's cool, but honestly trying to explain this on a message board discussion is just inadequate. Maybe there is a language barrier or perhaps I just haven't been clear.
"Thinking more about the actual chords and less about the key (which is common in jazzers)"
A common mistake, if you are talking about functional harmony. Regardless of your opinion on this, tonality considers the key paramount, by definition.
"I don't think you're making a fair point comparing major to minor... I7 and Im7 have nothing in common. "
Not true. Both m7 and 7 have clearly defined functional roles.
"Bb is not part of the key of C major but it's part of the key of C minor".
Harmonically speaking, this is not true in tonal harmony. When discussing harmony, B is the note expected in C minor, not Bb. When you consider the practical universe of each tonal key, there is less difference here than you think.
"Vertically a dominant chord is much harder to accept as a tonic (except on a blues) than a Im7 - the 3 b7 tritone being the main reason."
The tritone is problematic because of its conventional use in the dominant function. The min7 has a conventional function of predominant, so while the 7 chord may be slightly harder to accept, it is not "much" harder . . . and we do accept it, it just takes us into a modal territory rather than a tonal one. The existence of a min7 interval on a chord in functional harmony tells us that this chord is going to take us somewhere else and is not a point of resolution.
"In your voicings you absolutely never use Im7?"
If I want to push the harmony in a modal direction, I do.
" If you hear Im7 on any of the tunes we've talked you're ears go modal and loose the key?"
No, I don't lose the key, I hear the harmony shift to a modal, rather than tonal, focus.
"You hear Cm7 on softly as a IIm7 of Bbmaj7?"
No. I hear it as pushing the tune in a modal direction rather than tonal, Cm7 being the tonic of an aeolian, dorian, or blues sound rather than a minor-key sound. Which can be totally cool, but in that case I would probably downplay some of the other tonal elements (use just V7#9 or V7b9sus instead of iiø7 - V7, or create a modal sequence of some kind).
"And what about tunes that use the b6 on the I chord like "How Insensitive" or "Retrato em branco e preto"?
This is an interesting question, these tunes are a little different in that Jobim was both very interested in incorporating blues sounds and was working in a harmonic universe that was at the edge of tonal function in many ways. His harmonies tend to use a mix of chromatic elements and traditional tonal function. Also, he was composing his music very specifically . . . the extensions etc are a thought-out part of his composition, not embellishments added by jazzers.
For "How Insensitive", he wrote the first chord to be m7, but think about how tonally ambiguous that opening phrase is . . . it's not until the end of the last phrase that you can really say that the first chord was "I". It's just chromatically descending harmony and could have ended up somewhere else. The final chord I would play as m6, mainly because it voices better with the tonic in the melody.
For "Retrato", he wrote the chords as triads, but it's a similar situation . . . at the beginning, the Gm is not functioning as a tonic, so a m7 is appropriate (a m6 would work, but it would be kind of a "spoiler", as at that point in the tune it's not a resolution and we're not supposed to feel it as "I" yet). The final resolution I would play as m6.
You can think of it as asking "does this chord take us somewhere else, or is it the point of resolution?" The former makes mores sense as m7, the latter as m6.
I think that you are misunderstanding the limitations of what qualifies as "tonal" harmony vs. "modal". I am not at all saying that a m7 chord can't be a Im chord. I am saying it can't be a tonal tonic chord, that by definition a m7 chord is not at rest in tonal harmony.