In case you are still waiting for an answer after two weeks, I will offer some thoughts. The advice could also be used by others so here goes...:
Lens choice is going to depend on how far you are from the subjects. I would probably try to use the 24-70mm lens at the long end if at all possible. The reason is that either of the two lenses at the wide end (16 and 24mm) will tend to distort the subjects and most importantly expand the image: The people closest to the camera will look disproportionately larger than those on the back row (assuming you have multiple rows of people). At 70mm, the lens will compress the image a bit, making the size of the people on the back row appear to be closer to those on the front row.
The more rows of people that you have, the higher your F-stop should be. Stopping your lens down (higher F-stop number) will enable you to have both your first row and last row in focus. A higher F-stop number (F8 for example) will require that you either use a higher iso or put more light on your subjects. In the past, I would use a hand held meter, set my camera to f8 and 1/125 shutter speed (for example) and determine what my iso setting needed to be. Using a high iso on your 5dmkII is no problem.
As far as placement of the two speedlights, I would place them at a distance a little wider apart than the width of your group. I would place them to my right and left, but not behind me. The greater the distance from the subjects will enable you to get a more even light spread across the group. I would aim the lights to the opposite sides of the group in the middle of each half (for example if there are ten on each row, I would aim each light at the third person from each end of the middle row): the light from the flashes will cross each other. This will help to have all faces as evenly lit as possible. Using umbrellas would soften the light, but at the expense of flash power. And, unless your umbrellas are really big, you will not see much of a difference in the softness of the light if the flashes are very far away. I would not worry about umbrellas.
By no means is this the only way to do this. Maybe others have other recommendations.
I hope this helps.