Content structure, sections, page types: want single template but list is used

How should the content be structured to insure that all sections are recognized as leaf bundles with all pages are rendered with the single template?

For a content structure like this:

content/
|— charts/
|— |- charts.md
|— |- group1/
|— | |-g1chart01.md
|— | |-g1chart02.md
|— |- group2/
|— | |-g2chart01.md
|— | |-g2chart02.md
|— | |-g2chart03.md
|— posts/
|— |- posts.md
|— |- topic1/
|— | |-t1post01.md
|— | |-t1post02.md
|— |- topic2/
|— | |-g2post01.md
|— | |-g2post02.md
|— | |-g2post03.md

themes/
|— mytheme/
|— | |-layouts
|— | | |-_default
|— | | | |-single.html
|— | | | |-list.html
|— | | |-charts
|— | | | |-single.html
|— | | | |-list.html
|— | | |-posts
|— | | | |-single.html
|— | | | |-list.html

What are the expected results of nameing the section level files index.md instead of posts.md (or charts.md)?

What would be the expected result of placing a second file posts01.md at the same level as posts.md?

The site is rendering all pages with the single template as expected but then as the result of an arbitrary change, such as saving one of the content markdown files, posts.md renders with the list template.

Hugo 0.92

I’m not sure that I understand your question.

Path URL Template
content/posts/posts.md http://example.org/posts/posts/ layouts/posts/single.html
NA http://example.org/posts/ layouts/posts/list.html

If you want to create a section page for the posts directory:

hugo new posts/_index.md
Path URL Template
content/posts/posts.md http://example.org/posts/posts/ layouts/posts/single.html
content/posts/_index.md http://example.org/posts/ layouts/posts/list.html