I should have studied better before, I know, as basically in this forum there is all the info one needs, the problem is that it is sometimes difficult to spot - anyhow, the solution is in
this post: it provides a "pdfviewer.app" file developed on the Russian forum that I simply copied in "system\bin".
Then pdfviewer works (at least, so far it does!), and so does the dictionary.
So just to recap, it is possible to use the December version of 15.3 and all the applications provided that:
1) for a fully functioning dictionary in Adobeviewer you use the "hacked" version;
2) for a working pdfviewer you download and install JAW's pdfviewer.