After this, I will be a "run around" photographer at their prom (class officers of 2015 "hired" me), in which I will be getting candids and all that nice stuff....You'll want the Lightsphere for that part...and good luck with both parts of the shoot!
I agree, the Lightsphere will work best indoors. Another good diffuser that is lighter and less bulky is the Sto-Fen diffuser. I typically make sure the little white card is pulled out of the flash before I mount the diffuser to direct more light forward. Then I point my flash up at an angle to soften the light and get the light source up higher to minimize shadows behind the subject. This is what flash brackets do, they get the flash up much higher to further minimize shadows.
I can't wait to see your pictures! Good or bad, you will learn a LOT. The question is, when do you want to learn? Before (from practice) or after?
Keep in mind that the indoor prom pictures will require a much higher ISO than the garden pictures to look balanced with the ambient light. Don't make the mistake of relying on the flash to light the entire venue resulting in what I call 'crime scene photos' where the only thing that you see is the subject with a dim/black background and overexposed faces. You can get that result from any camera, phone or point and shoot. Use your skill and equipment to capture more interesting pictures. Again, practice. Good indoor photography is also a challenge and requires practice. Make sure you shoot RAW so you can fix light issues in post. Trust me, you will!
When shooting the prom, take some shots of the overall event but make sure you mostly shoot people, couples, and shots with emotion, laughing, expressions, etc. Think about what people will think when they look at the picture. Will it be boring (shot after shot of the crowd or group shots with tiny faces) or will they laugh and comment on this or that because it is easy to see in the picture (person A laughing with person B) where their faces fill the frame? You have to get close, you have to be bold and you have to get folks to relax. Take at least two or three shots. Make sure they didn't blink or have half closed eyes. Make them look good. Don't be afraid to say, 'You blinked!" and then ask to take another.
When shooting the prom and it gets pretty dark, you might end up using shutter priority to lock in the slowest shutter speed you can consistently hand hold and then take a lot of (high ISO, probably 3200) shots to hopefully get one that is in focus. Low ambient light is tough! Take plenty of shots and make some of them 'insurance shots' with flash, that you know came out ok even if they aren't super great and then take others that are more risky but have the potential to be great shots if you can manipulate them enough in post. I have always found that the shots look spectacular on the LCD but once you see them on your large computer monitor, you see all the imperfections. Noise, slight focus issues, blurs, etc. This is why you take a LOT of shots.
Take plenty of batteries, esp for the flash. Eneloop batteries work best and last a long time. Alkalines don't.
Sorry for all the lengthy posts but I've been where you are and I really hope you have a successful time! Please share some of your shots, both good and bad! Share the experience and what worked and what didn't! Good Luck!!