These are interesting and difficult situations. Neither one has the white dress exposed enough to be white. I downloaded both and checked with Photoshop for illumination at spots. The outdoor scene, the upper sky is 99.5% for each channel so that is the highest tone possible in the scene, which means the dress, unless illuminated at least as much is going to be underexposed to be white. Metering for the sky and setting that as several stops above the meter center, and using manual flash or TTL BL since it is a backlit situation, would have exposed the dress to full white. The groom's grey suit is a good mid tone. The interior shot has a problem with a closer subject, illuminated by flash from the right side being almost twice closer than the white dress results in the front right of the table cloth being 98% and the dress getting fall off from the flash, can't be white unless the foreground table cloth is intentionally over exposed. In a tricky situation like this, TTL will mess you up more often than not. By going to full bounce flash the effective distance difference between the white dress and the foreground would be very little so both could be exposed to white highest tone. Another way of doing that is to just move the flash back so the relative distance between the light and the dress, and the light and the table cloth, were closer together, by having the light further from both. In neither of these photos is there a misreading of the light by the meter, it got the high tone upper sky and prevented that from blowing out but it left a need for more power or closer for getting the dress white. The interior shot TTL flash got the foreground white so something almost twice as far from the light source had to be lower in illumination. So the meter and flash worked with what conditions they saw(The meter in the camera and the metering for the flash are two independent systems). If that table cloth was not there, the dress would have been bright white and the highest tone of the frame.
Regarding the white balance cap. It is fine for white balance when the light source is the ambient that illuminates the cap but using flash means whatever you set with the cap, will be confused by the mixed light of ambient and flash combined. The white balance cap is not a replacement for a 18% grey card which can be used for both white balance and for luminance checking, to have a mid tone reference using the same ambient light illuminating the card as the scene. Stan St Petersburg Russia