Q1 does not make sense. Bank capital is not "invested" in a specific asset such govt securities. Bank Capital is concerned with the Sources (not the Uses) of the bank's funds and is the amount that is attributable to Shareholders as opposed to Bank Bondholders or Bank Depositors. Hypothetically if all the assets of the bank could be liquidated, they could be used to pay back the Depositors, then the Bondholders and if anything is left the Shareholders. So Bank Capital is calculated as a residual amount in a (theoretical) liquidation.
Bank Capital is not bags of money stored in the bank which can be used to buy assets such as govt bonds or to pay bonuses, etc. Rather Bank Capital is the result of a calculation done by accountants following an agreed upon and regulated procedure. Do not think of it as a physical amount of money or you will get very confused; is it just a number, but a very important number.
Banks have made very risky loans for a long time (starting with the Medici, Bardi and Peruzzi in the Renaissance, and perhaps even earlier in China). Suppose the Medici Bank has a capital of 100000 scudi and they have lent 20000 to the king of England. The 100000 is on the RHS of the balance sheet as Capital and the 20000 is on the LHS as Loans. Suppose the king dies in battle and England defaults on its loan. The accountants recognize the loss by setting the loan value to 0 and (to keep the two sides of the balance sheet equal) setting the capital to 80000. The bank has less capital than before due to the loan loss. The reduction is a mathematical operation that occurs on the books of the bank as a result of the loan default.
That is the way it used to be done. The modern world is trying to take a more forward looking, statistical approach. Suppose we think the loans have a 5% chance of not being paid back. Then under a provisioning approach we would assume that 5% of the loans have already defaulted. So at the moment we lend to the king the capital would be reduced to 100000-0.05*20000 = 99000 scudi. The provision is 1000. Later if the king defaults completely we will take the remaining write down and the capital will go to 80000 as before. Or if the default is very minor (less than 5%) or none at all we may adjust the capital upward.
The idea is the same as before but we are tracking the amount of capital more accurately and dynamically by using estimates of the probability of default rather than the old yes/no default/no-default approach.