1) Can a set of transactions to a specified btc address be reversed?
Under the right circumstances, yes. It isn't easy to do, and it doesn't happen commonly, but it isn't impossible.
2) If so, how is that done?
There are a variety of known attacks.
One attack involves the attacker pre-mining one or more blocks that spends one or more inputs in a transaction to themselves, keeping that block or chain of blocks to themselves, then broadcasting a transaction that includes one or more of those same inputs in a transaction sent to the victim. Once the transaction is accepted by the victim, the attacker can release their private chain to the public. As long as their chain is longer than the current chain, the broadcast transaction to the vicitim will cease to exist.
Another attack involves gaining a network connection directly to the wallet that the victim is running. The attack sends a transaction that pays the victim directly to the victim's wallet. Meanwhile, the attack simultaneously broadcasts to as much of the rest of the network as they can a separate transaction that spends one or more of the exact same inputs but which pays themselves. The transaction that most of the network sees is most likely to get confirmed, which will cause the transaction that the victim received to cease to exist.
Another attack involves having enough hashing power to overtake the main blockchain from one or more blocks back. This will allow the attacker to confirm transactions that spend identical inputs to earlier transactions that they sent in place of the already confirmed transactions that they sent to the victim.
Another attack involves sending a transaction that is unlikely to ever get confirmed (due to extremely low priority and no transaction fee) to the victim. Then waiting for most peers to drop the transaction from their memory pool. Then sending a replacement transaction that spends one or more of the same inputs to a different address.
There may be a few more well known attacks, but I think that answers your question.
3) If so, who does it? - and under what circumstances?
It rarely happens. It can happen accidentally if someone sends a transaction that isn't likely to be confirmed. It can happen intentionally as per the previous descriptions if the attacker has the appropriate skills and resources.
4) If so, how secure is the process? Is it susceptible to a malicious actor?
Which process? Bitcoin's confirmation process is quite secure. The more confirmations you have, the more secure the transaction is.
5) What is the trigger for blockchain.info to show this double spend warning for an address?
Any time blockchain.info sees an input that is used in one transaction, and then later used in a different transaction, they mark it with a double spend warning. This can be due to orphaned blocks, unconfirmable transactions, or malicious actions.
6) Can I be sure (even after hundreds of confs) that a payment I recieve has really been recieved?
That depends on what you mean by "sure" and what you consider to be an acceptable level of risk. You're really getting into some philosophical concepts there. For example, can I ever be "sure" that anything other than my own consciousness "exists"?
7) Would you be so nonchalant if you were watching txns disappear from your own wallet?
If they just started disappearing when I hadn't done anything at all to my wallet? I'd be concerned, and possibly even in a bit of a panic.
If I had just done something significantly outside the normal use case for the wallet design (such as recovering an entire data directory from the recent past or attempting to re-download the entire blockchain)? I'd assume that I misunderstood the process and I'd calmly make an effort to better understand what I was doing.
8. How can I (if at all) defend myself from someone performing a double spend attack when sending me coins? (like the OP topic states)
Wait for a few confirmations. Be aware of any significant changes in the rate of new blocks. Be aware of any significant changes in the block orphan rate.
