The statement that 'Absent of defaults, the actual monthly payments are never higher than scheduled monthly payments' is false. This is because prepayments can occur, which are essentially early repayments of a portion of the mortgage principal by the borrower. Prepayments can speed up principal repayments and reduce the amount of interest paid over the life of the mortgage. This can adversely affect the amount and timing of cash flows from the perspective of an investor in Mortgage-Backed Securities (MBSs). Therefore, the actual monthly payments can indeed be higher than the scheduled monthly payments, even in the absence of defaults.

Choice A is incorrect. The statement that 30-year mortgages have lower monthly payments than 15-year mortgages is true. This is because the principal amount of the loan is spread over a longer period, resulting in lower monthly payments.

Choice B is incorrect. The statement that scheduled monthly payments are constant over the life of the mortgage is also true for fixed-rate mortgages. Since the interest rate does not change, neither do the scheduled monthly payments.

Choice C is incorrect. The assertion that actual monthly payments may vary over the life of a mortgage can be true in certain circumstances such as when there are changes to insurance or taxes which are often escrowed into mortgage payment, but this does not relate directly to whether a mortgage has a fixed or adjustable rate.