This trail is as safe as one makes it. Personally, I think it is probably safer at night. You won't have the sun beating on you, and you'll avoid the afternoon thunderstorms. The trail is very well marked. Get a good headlamp and climbing poles. I'd venture to say the most dangerous parts for falling is once you get above the switchbacks. You'll probably have daylight at that point, if you leave at midnight from the portal.

To answer your questions as numbered.
1. There are wild animals. Stay with your partner and don't get separated. Although very rare, Mt. Lions are a lot more freaky than bears if you ask me.
2. Trail hiking at night is as safe as you make it. Read #1.
3. I would not bring bear mace. Do your research and learn what to do if you face a bear. If you do decide to bring mace, prepare to be covered in mace. Every time I have used mace/pepper spray, I always end up being a victim of my own spray. Yes, I use is professionally!! Don't bring it.
4. The water source at trail camp is VERY easy to find. There are a ton of water sources as you hike up to trail camp. Trail camp is your last opportunity to fill up though. (Tip, just prior to trail camp, you will cross some water, I would fill up there. The water in the lake at trail camp is a bit dirty.)
5. It is easy to fall. I twisted my ankle on the first turn out of the portal. You're hiking in the mountains. To help with stability, train with hiking poles and have some good shoes (bring Ibuprofen for your pain).
6. I did the switchbacks in the dark. If you leave the Whitney Portal at midnight, you'll probably be getting to the switchbacks around sunrise. It isn't the darkness you have to worry about, unless your headlamp has failed, it is the elevation. You'll be going slow enough, believe me. Don't worry about the darkness!!

Insight: Figure out your average hiking speed on some elevation training hikes. If you are not training at elevation, you are going to be A LOT slower than you thought. Use your training hike times to determine when you should leave. We averaged no more than 1 MPH with our backs/breaks/photos/and messing around on the way up. Take your time, drink a ton of water, bring a lot of food, and have fun. If clouds start to form above the summit, prepare for lightning/rain. Be smart. Take some photos and post them back here when you are done.