Why can't we take the Ecumenical Turn?

I will now take it that Adams and Aizawa ought to concede that there is no difference in content between my thought that ‘the harbour Bridge looks beautiful in the sunlight this morning’ and my utterance to you that ‘the Harbour Bridge looks beautiful in the sunlight this morning.’ Indeed I might think that very sentence to myself in my head before uttering it to you. If Adams and Aizawa are happy to accept this conclusion, then we really have no disagreement, because we have an example of thinking in natural language (English in this instance). Similarly it seems obvious to me that a Venn diagram that I am imagining now has the same meaning as the Venn diagram that I am drawing on the page now.
     However, this ecumenical turn contradicts some of their earlier claims, where, for example, they “presuppose that thoughts have non-derived content, but that natural language has merely derived content.” (Adams and Aizawa 2008 p. 35) Given that commitment in conjunction with their stipulation, they cannot really help themselves to my example above. That is why I have always found Adams and Aizawa’s position so strange, it leads to clear problems and even contradictions.

I don't think the ecumenical turn contradicts some of our earlier claims; it is one of our earlier claims.  It really looks like Menary's view is that thinking in English is a matter of having a token of English that bears derived content occuring in the brain or a picture with derived content occuring in the brain.

Now, later, Menary writes this,

I suspect that it just turns out that Adams and Aizawa are making a merely trivial claim (which no one I take it denies): a pencil drawing of a Venn diagram on a piece of paper cannot literally be found in the brain—you wouldn’t find pieces of paper in my head if you opened it up and had a look. It doesn’t follow from this claim that the meanings of Venn diagrams aren’t the same whether they are represented as images neurally or as circles on a page.

Yes, we are making the trivial claim that no one denies, namely, that mental imagery is not a matter of little pictures bearing derived content being found in the brain.  Thinking in English is not a matter of sound streams bearing derived content being found in the brain.  So, Menary has to work out the idea of mental image and thinking in English in such a way that there are some other sorts of mental representations that have derived content in the mind.  That's the tricky part.

And, yes, it doesn't follow from this claim that the meanings of Venn diagrams aren’t the same whether they are represented as images neurally or as circles on a page.  But, A&A already said that mental representations and physical representations can have the same content.  That's what we are noting here:

The first thing to observe about the derived/non-derived distinction is that it concerns the conditions in virtue of which an object bears a particular content. A thought might bear the content that the cat is hungry in virtue of satisfying some conditions on non-derived content, whereas a particular inscription on a piece of paper might bear that same content by satisfying some other conditions on derived content. To put the matter another way, there are two questions one might ask of a representation. The first is what content that representation bears; the second is what conditions make it the case that it bears that content.  (Adams & Aizawa, 2010, p. 582).