URL/ToC issues when inserting one page into another

Regarding the TOC problem, you were on the right track with using the {{% %}} notation to call the shortcode, but there’s one piece missing. In the shortcode, you need to render .RawContent not .Content.

Assuming this structure:

content/
├── snippets/
│   ├── index.md
│   ├── snippet-1.md
│   └── snippet-2.md
└── _index.md

And this in your _index.md page:

{{% include "snippet-1.md" %}}
{{% include "snippet-2.md" %}}

Then the TOC is properly generated with a shortcode like:

{{ with $path := .Get 0 }}
  {{ with $.Page.GetPage "snippets" }}
    {{ with .Resources.Get $path }}
      {{ .RawContent }}
    {{ else }}
      {{ errorf "The %s shortcode was unable to get %s. See %s" $.Name $path $.Position }}
    {{ end }}
  {{ else }}
    {{ errorf "The %s shortcode was unable to find the snippets page. See %s" $.Name $.Position }}
  {{ end }}
{{ else }}
  {{ errorf "The % shortcode requires a single positional parameter. See %s" .Name .Position }}
{{ end }}