The nose cone for crew dragon protects not only the docking port but four forward Draco thrusters and the star tracker too. It has similar function to the Dragon v1 GNC bay door by protecting the GNC equipment when in atmosphere.
So the nose cone stays open most of the time to allow for navigation.
On top of that it looks like these four thrusters are the only ones that do not suffer from cosine losses which together with the thruster action shots captured from the webcast convinces me that they are the ones used for deorbit.
I think I see in this video the "flame" of 4 draco thrusters interacting, captured by the front facing camera under the nose cone (I think it is the same one that showed nose cone closing).
Source: NasaTV via @nextspaceflight
For the unprobable case when the nose cone fails to close or latch, it can be separated (probably with pyros? I did not find a definite source). The capsule can reenter without it (as Cargo Dragon does) but there would probably be some damage to what it was supposed to protect.