1. It's apparent that some exchanges have been hacked; my understanding is that it's due to weakness in their own security. Has the Bitcoin code itself ever been hacked (or the Blockchain in this case I believe)?
No.
I suppose an example would be the 51% hack?
There is no 51% "hack". The "51% attack" as it's called, is when a single entity obtain the majority (i.e. at least 51%) of the total hashing power world wide. There's a lot of misconception as to what someone with 51% could actually do.
Contrary to popular belief, they can NOT randomly create new bitcoins, they still have to abide the mining rules. They can NOT send fake transactions, or spend bitcoins they don't own. They can only
for as long as they manage to keep the 51% majority, which is VERY expensive, mine 100% of the coins (since their chain will be growing faster) and optionally, prevent other people's transactions from being confirmed by deliberately excluding them from their mined blocks. So it's not like Bitcoin suddenly collapses or becomes unusable or unstable or untrustworthy or whatever. And again, their advantage only lasts for as long as they are able to maintain their 51%.
Is it even possible to hack Bitcoin? I've never heard any talk about it so I've always assumed it cannot be hacked; impossible (suchas duplicating Bitcoins or manipulating in any way).
It's extremely, astronomically, tremendously, massively, immensely unlikely. We can't be sure that at some point in the future, some smartass will find a weak spot in the underlying cryptography, e.g. by breaking ECDSA or SHA256. However, rest assured, long before that happens, Bitcoin's security will be the
least of our problems (all other financial systems and money protocols use MUCH weaker security).
2. Bitcoin's competitors; of that there are many. (as can be seen on coinmarketcap; I wouldn't consider fiat a competitor..)
example NextCoin, DarkCoin, some other alternat Bitcoin copycats.
Do any of these other altcoins stand a chance against Bitcoin?
No. All of these altcoins
combined are not as large BY FAR as Bitcoin, in terms of acceptance, market cap, usability, level of software maturity, ecosystem, supported platforms, trading volume, business, start-up investments, amount of wallets and client software, etc.
Are they in a position to replace Bitcoin?
No. They offer nothing new, most of them are just a 'get rich quick' attempt from people who think they missed the Bitcoin boat and hope to create their own hype. Some small technical details aside, there's no significant novelty or advantage to any of these altcoins.
Does Bitcoin even need a replacement?
No. Bitcoin isn't even finished yet, even though it works great in every day practice, technically it's still in development. Any new features or suggestions or improvements that would be introduced by some altcoin, could -if it's necessary or worth it- be integrated into Bitcoin as well.
