###Flash: If the light is not enough then you need a flash! If you have a full-frame body that can shoot high ISO with not so much noise then you might not need the flash but generally speaking indoor photography in the night needs flash.

You should point out the flash to the roof IMHO and let the reflection from the walls and roof light the environment.

###Focal length: Depends! If you are interested in people attending the event, then you need a zoom but also, it's always a good idea to have some wide angle shots for indoor. It gives a sense of the size and tells the story of the event. Long story short, you probably need both.

###On interruption: You probably will interrupt anyway! If you have to document the event, then don't bother yourself about interruption. You need a flash and your flash would be interrupting. If you are going to fire your flash every often, then forget about the interruption and stick to your photography duty. In other words, if you think you are going to have a good shot but you are afraid to interrupt, don't worry and take your photo! Obviously this last statement doesn't hold for privacy of the people attending the event.

###What else? I can suggest you three things.

Most obvious: Shoot raw. Secondly,

it would be a very good idea to go to the place prior to the event and study the place, find your spots and plan your photography session.

If you are going to have group photo at the end find a good location for that as well. This is one of the very first things that wedding photographers learn. Finally, depending on the type of event you can shoot your photos with a warm tone (this is a matter of post processing if you shoot in raw). If you look at the commercial photos of restaurant they all have a warm toning to created a sense of friendliness. However this last suggestion is more a matter of personal preferences.

Flash:

If the light is not enough then you need a flash! If you have a full-frame body that can shoot high ISO with not so much noise then you might not need the flash but generally speaking indoor photography in the night needs flash.

You should point out the flash to the roof IMHO and let the reflection from the walls and roof light the environment.

Focal length:

Depends! If you are interested in people attending the event, then you need a zoom but also, it's always a good idea to have some wide angle shots for indoor. It gives a sense of the size and tells the story of the event. Long story short, you probably need both.

On interruption:

You probably will interrupt anyway! If you have to document the event, then don't bother yourself about interruption. You need a flash and your flash would be interrupting. If you are going to fire your flash every often, then forget about the interruption and stick to your photography duty. In other words, if you think you are going to have a good shot but you are afraid to interrupt, don't worry and take your photo! Obviously this last statement doesn't hold for privacy of the people attending the event.

What else?

I can suggest you three things.

Most obvious: Shoot raw. Secondly,

it would be a very good idea to go to the place prior to the event and study the place, find your spots and plan your photography session.

If you are going to have group photo at the end find a good location for that as well. This is one of the very first things that wedding photographers learn. Finally, depending on the type of event you can shoot your photos with a warm tone (this is a matter of post processing if you shoot in raw). If you look at the commercial photos of restaurant they all have a warm toning to created a sense of friendliness. However this last suggestion is more a matter of personal preferences.