How to substitute oranges in a mango mousse recipe?
You might take a strategy from a orange olive oil cake that I make.  Cut about 1/2 inch off the ends of oranges.  Then quarter the oranges.  Place them in a pot of water, then bring to the boil.  Drain and repeat 3 to 4 times.  This reduces the bitterness of the pith.  Now, add 4 cups of water and 1 cup of sugar to the pot.  Add the oranges and bring to a simmer.  Cook until they are tender, 30 to 40 minutes, then drain (you can reserve the light syrup for another use).  Blend the cooked oranges in a food processor or blender.  Use 450 grams in your recipe (strained), eliminate the lemon.  Let us know (with a comment, or an answer to your own question if you choose a different approach) if it worked.