It looks like this Theme Component isn’t being updated much these days, and the to-do list seems to perhaps include my problem (DiscoToC), but I thought I’d mention it anyway in the hopes it will inspire some updates here. 
It appears that Blog Post Styling and DiscoTOC are fundamentally incompatible, at least with the site width I have. The ToC overlaps the text and I haven’t found a clear way to address this with CSS alone (though I am far from a CSS expert!).
I tried using the Default theme and removing my CSS customizations and it still happens. But as soon as I disable Blog Post Styling, it is resolved and the ToC appears in the right place. I would really love to be able to use both together though.
I have disabled DiscoToC for now, but you can see my site (with customized theme) here if that is helpful:
If it is helpful for me to re-enable DiscoToC so you can see this happening live, let me know.