I mentioned at the lecture yesterday that for our purposes you can ignore interest rates in the estimation of the market price of risk.
For the last question, since X1/X2 is singular when the conditioning event is {X2=0}, rather than plotting the conditional expectation of X1/X2, plot the two curves given by the conditional expectation of (X1, X2).
For the third question on the fall assignment, you can assume that all of the variances are one.