For the redundant buoyancy question, you first need to ask How many BC's have you seen with a 100% loss of air?
I have seen a few loose air 100% because the hose came unscrewed, the solution was to screw it back on or have your buddy do it. Personally, I have had a dump valve spring go, and came up no problem by keeping an attitude in the water to keep that valve low (on my side). Both of these failures are checked for pre and post dive as normal maintenance. What I do now is at the end of each dive day I rinse off the BC, blown it up with the power inflator till at least one of the dump valves lets go, then rinsed off again, dry, deflate, and store. Then just before a dive and after the rig is assembled, the BC is checked to make sure the hoses and valves are tight and blown up and checked for air hold, inflator, and valve operation.
I have punctured a wing now and then, but with a good quality BC cover and bag is very hard to slice open. A puncture will bleed air, but not to any large amount that cant be controlled by oral power inflator additions. As for BC's getting a slice, I have never seen a slice, but I have seen cuts caused by a pinch. I had a Diverite Classic where the BC was on a set of tanks which were put into a van laying down and then other tanks were put on top. One of the top tanks had a pony attachment band that made the top of a pinch between it and the 72. The result was a 1" pinch which I did not notice till two or three dives later when it was pointed out to me by another diver on a deco hang. The pinch was low enough that it just was not a bother in most positions. The tanks I was using at the time were HP 100's.
The worse BC failure would be a torn inflator hose or attachment mount, and I have never seen one of those. In open water, a bag to the surface or going up an up line/anchor line is an easy way out. But, deep in a cave with a long horizontal distance to cover you might be into some deep dodo.
As for dry suits, I come from a time when if you had a dry suit you did not have a BC. I make sure I am weighted correctly for the end of the dive when I will most likely want more air in the suit for comfort/warmth in the DECO stage when you just are not using the big muscle groups and generating a lot of heat. The BC gets dumped at the surface and no air gets put into it till I'm back at the surface.
As for how much can a dry suit lift, a lot. Get in your gear with full tanks, dump your BC inflate your dry suit till the neck seal starts to burp and have your buddy try to push you under the water or hand you weight till you sink. I think you will find it will be close to 25-30#+.
For loss of buoyancy, the worst suit I ever used was the Poseidon Uni-suit which is a full expanded neoprene dry suit. It has all the good and bad characteristic of a dry suit and a wet suit combined, but can be very warm in real cold water.
Pete Johnson
The following is from Joel Silverstine a while back:
B A L A N C E ..... balance.
Much of the "dont dive steel with wetsuits" information came out of South Florida in the mid 1990s when DIR / GUE / WKPP programs were being created. Much of this had to do with diving PST 104s with 3MM wet suits .... that in and of itself is a big ole problem larger than we have room to argue over. Since they were not going to change steel tanks aluminum was the logical choice.
Steel cylinders have been in use with wetsuits much longer than aluminum cylinders have been. Go back to the beginning before alum cylinders were made for scuba.
Different steel cylinders have different buoyancy characteristics. Some are rock heavy, some are light as a feather (loosely speaking) 7, 7.25, 8" cylinders all are different and displace different amounts of water. And each diver is different in displacement as well.
Remember that a BC / Wing is not an elevator it is a "compensator" it is to compensate for the decrease in thickness of an exposure suit and the compression of the air in the wing and lungs as one goes deeper. In an ideal world all you really need is enough lift to make you 1 lbs positive and you can then swim up. But the real world does not work that way and we have to sometimes overcompensate for stability with additional ballast for control.
My favorite cylinder with a 5mm wet suit in fresh water is a Faber 95. With a 3mm a Faber 85. I can glide -- move to salt water and I will add a little weight to compensate for the salt water.
For doubles it gets a little more involved because of the weight of the bands and manifolds but still manageable.
I have found that Faber LP steel cylinders have a cleaner buoyancy swing than the X cylinders have. And when not overfilled they balance out very nicely with wet suits.
Aluminums tend to be a bit floaty and will require 8 lbs of neutralization balast to be added for just the cylinders.
Assuming the balance of the rig can be created and, we know it can the big question comes down to this.
"What is the probability that a dive wing will fail and provide zero lift?"
Since 1989 when I started to dive with wings. I have had 3 wings provide some problems in
Wing 1. Dive Rite Classic wing - 1800 dives on it developed pin hole leaks at the welding seam. (did not lose buoyancy - replaced cell)
Wing 2. Halcyon Single Wing - 20 dives on it snagged wing on piece of wreckage on the Rhone and got small hole. (Halcyon could not repair or replace - I threw it out)
Wing 3 Inflator cracked due to HP seat failure in regulator. Wing held air as hose was below the bubble level.
Dive equipment rarely fails. When it does more than 95% of the time it has to do with a maintenance issue which falls back to user issue.
Divers need to learn how to inspect equipment pre and post dive. This will lead to few if any failures. Care for equipment on boats is important as well. Keep your gear tidy. In your space and not all over the place and it will rarely if ever get damaged.
So to answer the OPs question. .... Yes you can dive steel tanks with a wet suit so long as the rig is balanced and you can swim it out should you have ZERO ability to inflate the wing. The balance is something that is based upon your size and needs and the needs of your team.
cheers
JDS
) frequently. 
