Giulio,
There is way to mirror and keep the features apart from the parent part, as I have demonstrated below. I mirrored the below part and kept all the features. This was performed under Save As>Mirror. So the software will work, but I couldn't do this with your part, as there are some features within your model that cannot be mirrored. Try suppress everything but your first feature and see if you can mirror it.
Figure 1
Figure 2
Figure 3 Mirrored Part