Incorporating Italian meringue into mousse
I'm not sure it's possible to overbeat the meringue (not without falling asleep while doing it), so I don't think that could would be the main cause of a failed mousse.

On the other hand, it's definitely possible to overbeat the cream for the purpose of being an aerator, and it's a very common issue. For mousses you don't want cream to get to stiff peaks: it should be shiny and not hard. This also helps to avoid overdeveloping the cream when folding it in.  If you need a visual reference, this video could help.

As for order of mixing, you should generally mix softer things into stiffer things, so I would do whites first and cream last, and for each add it in three separate steps, folding them in gently.

One final but vital thing to consider to avoid graininess in a mousse is the chocolate itself: a chocolate with too much cocoa butter will result in a grainy, unpleasant texture - a chocolate with too little cocoa butter will yield a mousse that is too soft and not able to stand as a layer in a modern entremet. Some brands of baking chocolate will explicitly say which products are the most indicated in a mousse.