I think your mousseline is fine from what you write. Mine’s often looks curdled or soupy while adding the butter and/or towards the last drop of butter, but then it comes back smooth after a fast final whip, this is when the magic of perfect emulsification happens.
Personally, I don’t think you can over whip mousseline. It is really a physically strong buttercream even after the flavor additions. HOWEVER, I can’t stress enough to heat the sugar to the correct temperature in the recipe.
By the way, I am experimenting by using natural fruit juice instead of water when making the sugar. Yesterday I made my Baked Hawaii 2009; when whipping the pineapple italian meringue, the flavor of pineapple was so great that I think why not use it for mousseline! You could do the same with lemon juice, orange (similar to the lemon Neoclassic Buttercream).