The footnote in the attached epub pops up for me when sent to Kobo as a kepub. I used the same method I always use for footnotes.
Where you have used 2 adjacent <a> tags, one for the id and the other for the href:
Code:
<p class="calibre32"><a id="..." class="..."></a><a href="..." class="...">1</a></p>
merge them into one containing both the id and the href:
Code:
<p class="calibre32"><a id="..." href="..." class="...">1</a></p>
... and the same for the footnote, change
Code:
<p class="firstnote"><a id="..." class="..."></a><a href="..." class="...">1</a>-3. La poesia di Dante si avvia a cantare un pił sereno argomento, lasciando l’angoscioso mondo infernale.<a href="..." class="...">←</a></p>
to
Code:
<p class="firstnote"><a id="... href="..."" class="...">1</a>-3. La poesia di Dante si avvia a cantare un pił sereno argomento, lasciando l’angoscioso mondo infernale.<a href="..." class="...">←</a></p>
If your book is coded consistently and you have some regex skills you should be able to fix the book quite quickly.
I'm not going to claim it's the best solution or the only solution, but it's always worked for me.