In any lens, elements are the specific lens pieces used and how are they arranged to obtain a particular feature, the arrangement is the group. Lenses pieces are usually arranged in groups. In lens, say if it is a zoom lens, it will have more number of elements and groups, where as for a prime lens, it will have less number of groups and elements. 

Usually it is termed as the more number of groups and elements, the better the quality but it is not always true. Lens quality also depends on the type of glass, design mechanism, manufacturing process of the lens. So for a particular focal length, a prime lens would be better than the zoom lens.

Prime lenses usually perform better than zoom lenses.

First of all that you should not try to disturb the audience. Always try to capture pictures from roaming around the walls of auditorium. Better to go with Telephoto for taking candid shots for stage subjects or from the audience.

Another thing is that you can use another camera for wide angle shots of entire stage or the audience.

there are 2 ways of achieving focus

Using Autofocus

turn on the auto focus mode on your lens->then point at your subject->click shutter release button half way to lock the subject in focus->adjust your frame if needed WHILE HOLDING THE SHUTTER RELEASE BUTTON->press the shutter release button to take your photo with subject in focus

Using manual focus

turn on your manual focus on the lens->point at your subject->use the focusing ring to adjust focus according to your eye->press shutter release button to click the photo

1. Shooting with 50mm lens can let you shoot at aperture as wide as 1.8 but a 18-55mm will allow you to shoot only at 5.0 aperture 
2. This difference itself is a huge deal. This means you can have the advantage of having faster shutter . Imagine shooting handheld at 1/15s with aperture f5.0 and then at 1/125s with aperture f1.8. This can be significant in low light conditions.

f1.4 gives faster shutter speed as compared to f1.8. The former could be helpful when a narrow depth of field is required or when shooting in low light. Sharpness of the image at f/1.4 is more than that at f/1.8.

Wedding being a milestone in the lives of the bride, the groom and their respective families, they want natural emotions captured during the course of event. Some guests find it uncomfortable to pose for pictures. Candid photographers take pictures without making the subjects aware of their presence. So, people go for candid setting to capture the unadulterated emotions.

On the other hand, in a traditional photography setting, people are asked to pose for the camera. A lot of pictures captured in a traditional arrangement are for the wedding albums.

I usually format my cards before every shoot. Make sure you have at least two copies of the same saved as back up. Name a folder in camera itself and then start shooting. 

It depends on where are you uploading your pictures. Irrespective of the place, you watermark must be small enough not to cover much of your photo. You can keep your watermark mostly around bottom corner.