The "group" header doesn't add anything semantically relevant. Are you doing this purely for display purposes?

The simplest thing that is semantically equivalent would be just:

Join[data]
(* <|"on" -> <|"A" -> <|Length -> 19, Total -> 8|>, "B" -> <|Length -> 19, Total -> 8|>, "C" -> <|Length -> 7, Total -> 0|>|>, 
     "off" -> <|"A" -> <|Length -> 9, Total -> 42|>, "B" -> <|Length -> 7, Total -> 11|>, "C" -> <|Length -> 9, Total -> 27|>|>|> *)

Or wrapping that with Dataset:

If you really want the "group", you could do something like this:

<|"group" -> #|> & /@ Join[data]

or with Dataset:

Dataset[<|"group" -> #|> & /@ Join[data]]

If you really want the Length and Total headers to be at at the top under group, then it would take me more time than I'm willing to spend fiddling with it. Dataset has very strong "opinions" on how things should be displayed. Maybe just transposing Length-Total with A-B-C would work. If your main concern is just display, then I'd use some other structure, maybe even just Grid.

Could you also add the exact output that you want to achieve? I am not sure of what you want to do, but I am sure it is easier than what you are doing.

Hi, I have corrected the "on/off" key and the missing |> in data now (last minute edits). But the final dataset still generates a duplicate row?