The obvious answer (although I don't know if it matches your specifications) is to have the future Louis II die between 1889 - 1898 before he gets stationed in Algeria and has a chance to conceive Charlotte but after Albert's marriage to Alice Heine. When the OTL anti-Grimaldi sentiment hits (around 1911) you'll have an aging and unpopular prince with his German nephew as heir and in response you might see an equivalent of the OTL 1918 treaty, here giving Monaco to France upon Albert's death.