Abseiling questions

[longlivehumour]

Abseiling questions that are too elementary / naïve for Google to be of much help:

• What happens if you panic and let go of the rope? I know there's a device that prevents you from plummeting to your doom, but at what point does it kick in - do you drop a few centimetres, half a metre, more? Do you spin? How easy/difficult is it (in practical terms not psychological) to resume your descent?
• Apparently long abseils and potholers use something called a brake bar rack, a device for controlling descent. I watched a few videos and have a vague idea of how it works, but are there e.g. any bits you should never let go of when using this method?
• Is military equipment significantly different in this regard?
• If abseiling with others, how do you prevent your ropes from getting tangled? (Unless sharing a rope is an option, in which case whoopee.)
• What other minor problems might arise during a descent? Assume equipment is in good condition and was set up by someone who knew what they were doing.

Context: character is abseiling freely into a deep (100-150m), very wide cave (no wall-hopping), accompanied by three others. All are using specialised military equipment. The rope is safely fixed near the roof/mouth of the cave, with people watching over it at the surface. Speed is not of the essence: they just need to reach the bottom without being pancaked. They also can't descend one by one, as there's a chance those left behind might be arrested (long story). Wind may be an issue: it's a closed cave, but with a very large space and several small openings to the surface.

Character is a soldier who hasn't practised in years and is currently discovering a previously-unknown fear of heights, which is not a good thing to discover when you're 120m above the next solid surface. He's not going to go over the edge (ha) but I do want to have an idea of what can happen to make him twitch, besides my overactive acrophobic imagination.

[crossposted from TV Tropes]
Research (from this):

"Clients should be aware that the weight of the rope will be felt very strongly once the Rack is clipped to their harnesses… The weight of the abseil rope means that clients may find it hard to lift, and they will not be able to move down the rope unless they lift the rope. The short answer is that the client will have to have the stamina to lift the rope many times in order to complete the abseil. […] Correct manipulation of the rope will also allow the Rack to slide even with great weight (thus it is possible for two people to abseil simultaneously a short distance apart on a long rope with Racks). "

As far as I can see, the chief obstacle to sharing a rope, besides safety reasons, is that each person adds to the weight, and this slows down descent by friction on the rack. Am I right? Any idea on whether it's possible for four people to share one rope?