Set URL Parameter Post Layout As Default

I want this to apply to all posts

First off, making the content query string defaults to onepage is actually equivalent to enabling the onepage() for all URLs/pages.

And for example to enable it by default on single post pages only (for any post types), then you can replace this:

if( empty( $_GET["content"] ) || "onepage" !== $_GET["content"] )
    return;

with this:

if( ! is_single() && ( empty( $_GET["content"] ) || "onepage" !== $_GET["content"] ) )
    return;

which means the onepage() would also be applied on pages/URLs where the content=onepage presents in the query string.

Check the developer docs for other conditional tags like is_singular().

Does that answer your question?