The nacelle coming apart is not supposed to happen - design intent is that the nacelle is designed to withstand the forces of a FBO event without failing. Further, large parts departing the aircraft is a no-no - corrective action should be taken whenever it happens.
Well, it all depends upon the actual unbalance load. The normal assumption is one full fan blade, including the retainer portion which fits in the fan disk, a rather big chuck of material. From the pictures it seems the first blade ruptured just above the spinner so this portion of the blade stayed in place. The departing ('liberated') blade probably took away the adjacent one, at mid span. Not sure if it results in unbalance loads are higher or lower than the nominal case. The engine RPM is also an important parameter.
The FBO demonstration during engine certification typically demonstrates only one fan blade will depart. This is a big deal, because it directly drives the sizing of the containment ring and dermines the unbalance loads used to siez the airframe, nacelle included. The data I remember on such engines is that the separation of 3 adjacent fan blades, at 100% RPM, will result in loads well above the front spart capability of the wing, resulting an instant catastrophic failure of the aircraft structure.
Compared with the certification test involving a full fan blade, in service events with partial blade separation often result in multiple blade damage (and eventually rupture), because the cinematic is different, but the certification case is expected to be the envelope. This will be confirmed (or not) by the investigation.
It is interesting this involves a hollow titanium fan blade design. A similar material is used on the Trent and it resulted in various events with ruptures just outboard of the spinner.