I see two solutions.
1. Build your own version of Sigil, in which the index file will have the h1 tag.
All you have to do is correct two lines in file /src/MiscEditors/IndexHTMLWriter.cpp
Here and
here.
And one line in file /src/BookManipulation/Book.cpp
Here
But of course I understand that this is not a solution for everyone.
2. Replace (only once) the entry in the css file for the index:
div.sgc-index-title to
h1.sgc-index-title
Instead of manually replacing div with h1 – use the plugin.
Based on a
simple example prepared by Doitsu.
If you use a non-English version of Sigil, replace the word "Index" in the plugin.py file (line 9) with the translated one.
Maybe someone will come up with other solutions?