Montgomery County may provide by local law for the county’s guarantee of the principal and interest on bonds that the Montgomery Commission issues: to finance the acquisition, provision, development, or rehabilitation of housing at rental rates and prices not being offered in adequate quantity by the private sector; or to finance wholly or partly: mortgage loans secured by the housing; and fund-related reserves and costs that are approved under this subtitle. When a mortgage loan is financed partly by bonds guaranteed by Montgomery County and partly by bonds not so guaranteed, the part of the mortgage loan that is financed by bonds not so guaranteed shall be insured by: the Federal Housing Administration; the Maryland Housing Fund; a private mortgage insurer that is: authorized to do business in the State; and approved by the Federal National Mortgage Association or the Federal National Mortgage Loan Corporation; or more than one of these insurers.