A fast 50mm would have worked great here, if only you had that sigma 1.4
The problem when shooting a small venue is that you dont always have time to switch lenses and would normally have a wide to medium telephoto zoom lens on (24-70, 17-55,...). The 50 would be too tight in a small venue for alot of shots, so I would not think of using it.
This is where some experience might be handy... personally, I would not chose to shoot from an area where there is a high traffic place in back like the buffet table. Same goes for a door where people can come in and out of. Be mindful of the background at all times, plan where you are going to shoot critical times (like this dance) from. But keep in mind that while you plan, **** is still going to happen and you need to be able to adapt quickly.
I dont think a wide aperture would help that much more. The flash with a fast shutter to reduce the background might help some. But really, planning is probably the best thing.
When walking into a venue, take note of the main table, the door, the dj, chairs, setups, lights, exit signs (so you dont have to clone them out) and pick and chose shooting places.
I probably would not include that in my set of images, but again, it depends what else you are giving them. If they have plenty nice ones from the dance, might want to skip this one. If you feel this adds something to the story of their day based on your other images, add it