The short answer to your question is, YES, there is significant cell death ocurring in each of the conditions you mentioned (some more than others). I've been out of the field for a little over a year, and there has been data recently suggesting that certain cells in the nervous system can reproduce, BUT, I believe the current thinking is still that information storage (long term memory) in the brain is a combination of the physical pattern of axon-dendrite connections, their firing patterns, and the current biochemical state of the brain. The connection pattern is of primary importance and if it is disrupted, information is lost. Luckily though, the brain seems to have a holographic method of storage which retains memory across many millions of cells at the same time by repeating connection and firing patterns to create a robust network. So memory loss is indicative of a system wide condition where many cells are dying.
Alzheimers is an excellent example of this type of memory loss - upon autopsy brains are actually seen to have large holes where cells have died. Stroke is another example of significant cell death, but localized to a specific area. Stroke victims often incur many undetected micro-strokes in different areas of the brain which can give them the appearance of gradual memory loss. Brain damage through blunt trauma can take many forms, but the most common type (as seen in accident victims) is destruction of large areas of tissue through swelling inside the cranium. Damage by piercing or slicing wounds is survivable in certain cases and can produce the type of effects you were suggesting. When certain lines of communication are cut off , there can be a loss of signaling between those areas resulting in a loss of access to data by critical areas of the brain. Another example of this type of condition is amnesia, but as far as I know the mechanisms for most physiological types of amnesia are still not completely known, but loss of signaling is certainly implicated.
As to your other question, cells (esp. neurons) very rarely absorb significant partial structural damage and survive. Either there is damage that can be repaired to bring a cell back to nearly full function, or it is going to be destroyed by apoptosis. Molecular damage is another story, and can have more gradual effects (heavy metal poisoning, oxidation, etc.)
Hope that helps. I know others on the list have a strong interest in connectivity (Anders, Eliezer, etc.) and may be able to answer more completely than I. (I was primarily a molecular and cellular guy and didn't get to think about system wide properties very often) A good introductory book to these issues is Bill Calvin's "The Cerebral Code".