None of which is true.
Multiple warhead missiles first appeared in the early 1960s. Poseidon, Polaris, and Minuteman IIIs were all MRVed and later MIRVed. The Minuteman III was MIRVed from the start. It was cheaper to have one missile carrying several warheads than to have several missiles carrying one warhead. It had nothing to do with a first strike capability and everything to do with the US having superior technology. MIRVed warheads was a natural progression of MRV. MIRVed warheads are also much harder to protect against with ABM systems.
For more information on MIRV