Is there a supported manner to insert anything (ads in my case) at a certain point within the content, say 1/3rd from top, mid-way etc? I know it can be done within each .md file, but I am looking for the single.html template to handle it.
Also, it should take care of not inserting the ad in between the letters that form a word. It should always insert the ad in between words or sentences.
No, you cannot do this without a shortcode.
Shortcodes are definitely the best way to do this within individual content files.