Did Mendel believe that proteins are hereditary substances?
The first person to directly indicate a linkage between the hereditary material and enzymes was Garrod in 1902, based on the observation of the hereditary enzymatic disorder alkaptonuria. This is after Mendel's death.
The tetranucleotide hypothesis (i.e. DNA is not informative) that you refer to seems to have been formulated around 1910. So there is no way that Mendel knew about that.
It was not even proposed that proteins were composed of amino acids before 1902. No protein was sequenced before 1950. So Mendel had no idea what sequences or amino acids were, for any biopolymer.
It is formally possible that Mendel knew at some point about Miescher's result, but I'm not aware that he was at all interested in the chemical nature of Mendelian factors (what we now call &quot;genes&quot;). I agree with commenter @jamesqf on this point.
It was not immediately obvious to anyone that there was a discrete mapping between phenotypes and specific agents such as proteins. The major theories of inheritance at the time came from biometricians such as Galton, who believed that inheritance was driven by a blending-like mechanism (more or less), based on phenomena such as regression to the mean. They weren't really interested in the chemical basis either.
Mendelian genetics was only rediscovered around 1900 (Mendel was basically ignored during his lifetime), after which the debate started to consider the question of discrete genetic factors.
In short, I don't think that there is any evidence that Mendel had any interest in chemistry or the chemical relations of the organismal characters he studied.