How to divide a page into two columns to create an index, without that structure being applied to the entire document? Or is there another way to create an index?
When you create an index / table of contents, in the dialog there is a Columns tab where you can set the number of columns for the index. With an existing index, right click on it to edit it and then select the number of columns.
You can also use different page styles or sections to have different number of columns in different parts of the document, but if you only need columns for the index that’s not necessary.