Thanks on the explanation 1script. This means that your question is not really focused on a mobile site and js lazy load, instead the question is valid for both, desktop and mobile:
How (and is it) the content loaded via js lazyload attributed to the parent page?
You say you would like to keep the same URL, this would therefore exclude having a separate hash bang (#!) fragments as they would be treated as separate URLs from crawling and indexing point of view.
I am not sure myself, and I am speculating here that perhaps including the <noscript> element which would include the whole page content may do the trick?