I don't think I'd consider myself an "expert" on these topics, but I'll offer what I can until someone with more knowledge joins the conversation:
-Why was 21 000 000 choosen as a "maximum" for the number of bitcoins?
The first ting to realize is that the actual number as far as the programming is concerned is 2 099 999 997 690 000.
I think that number is just a byproduct of the inflation schedule chosen. The decision to start with issuing 50 BTC (5 000 000 000 as represented in the programming) per block was likely an arbitrary choice for a reasonable number that people would be likely to accept. This decision was made entirely by the person or group known as Satoshi Nakamoto, and I'm not sure if he/they ever gave any specific reason why that number was chosen. Next a decision was made to reduce the amount issued per block approximately every 4 years. Again, this was an arbitrary choice made entirely by Satoshi Nakamoto. I've seen some indication that 4 years was chosen as an amount of time for the markets and users to adjust before reducing again, but I don't believe there was ever any specifics given as to any calculations to justify the decision. Finally a decision was made to reduce the amount issued by cutting it in half and truncating any decimal (as opposed to any other formula that might be chosen). I strongly suspect that this decision was based largely on the ease of such a calculation in the programming. The reduction can be simply accomplished by performing a "right-shift" operation on the binary representation of the amount issued. When you work out the total after following that inflation schedule, you end up with 20 999 999.9769 BTC ( 2 099 999 997 690 000 as represented in the programming).
Note that the actual final amount isn't really important as long as it is adequately divisible. Less units issued would result in higher value per unit, which would simply be divided into smaller pieces to represent smaller values. More units issued would result in lower value per unit, which would be divided into less pieces to represent the same smaller values.
-Why the rate of Gold? In other words, were people behind Bitcoins interested in bringing back the Gold Standard?
I'm not sure what you mean here. Bitcoin is not tied to the exchange rate of gold at all. Clearly, the person or people behind the creation of bitcoin were interested in a currency that could not have it's inflation schedule arbitrarily changed to suit the whims of whatever government happens to be in power at the time. It shares this attribute of limited supply with gold.
-Despite optimism saying that the government cannot do much for Bitcoins...In reality, what can it possibly do?
The government? Which one?
I'm sure there are many things
a government could try to do if they wanted, but it is difficult to predict how successful any such efforts might be, and whether or not the effort would be worth the associated costs.
A government could attempt to affect bitcoin miners by either overwhelming the system with their own government financed mining operation. It would be quite expensive to acquire enough hashing power to truly overwhelm the system.
A government could attempt to affect bitcoin miners by creating and enforcing laws that affect the mining process. This might drive mining out of the country, but miners would continue to operate in other jurisdictions where laws affecting the process either haven't been created or aren't enforced. A large government such as the U.S. could attempt to leverage their relationship with other governments to get those other governments to enact and enforce similar laws, but it isn't clear if they could acquire enough compliance to significantly affect mining in the long term.
A government could attempt to affect price stability buy randomly purchasing or selling large quantities of bitcoin at the most popular bitcoin currency exchanges. Each such cycle might reduce the population's faith in the ability to reliably use bitcoin as a unit of account. However, those speculators who took advantage of the large swings in price would make a large profit at the expense of the government. The government might delay the adoption of bitcoin as a currency, but would significanly increase interest in bitcoin as a speculation and short term investment vehicle.
A government could attempt to stop the use of bitcoin within their own borders by creating and enforcing laws that make it illegal to use or accept bitcoin as payment. Meanwhile the use of bitcoin would continue outside that government's borders and a "black market" would likely form within the borders for those who are willing to risk prosecution.
Those are some of the most commonly stated potential government attacks. There may be others that I haven't seen or have forgotten.
-Could a government actually control prices by buying and selling bitcoins? In theory, yes but doesnt that show that bitcoins could possibly be a threat to itself since the government could easily manipulate the currency to make it more unstable?
At the moment the total valuation and market liquidity is such that a large enough government that is willing to spend enough could potentially control the exchange rate. As bitcoin gains popularity and usage, the liquidity in the currency exchanges and the total valuation will increase. It is likely that it will eventually become extremely expensive for a government to try to maintain such controls. To do so might require such a large portion of the government's resources, that they'd destroy their own currency through hyper-inflation in the attempt to create enough of their currency to influence the bitcoin market directly.
While we wait for additional responses from other "experts", let me know if anything I've said is unclear, mistaken, or needs additional discussion.
