1. Bit of a disadvantage there compared with the Soviet program which could reuse R-7 pads in just two days.LV reliability was improving by 1964 but still potentially an issue and the standardized Atlas SLV core had not been adopted across the board yet; Mariner 3 and 4 were still using custom-built Atlas D vehicles tailored for the particular mission needs.LC-13 was used for Vela in 1963-65 with the sole exception of Mariner 3 while LC-12 was used for NASA missions--Ranger, OGO, and Mariner as well as the FIRE Apollo heat shield tests but each of these required the pad to be reconfigured (also note that 15 months passed between Ranger 5 and 6 so LC-12 didn't get used at all during 1963). Plus of course Ranger used Agena B while Mariner used Agena D which necessitated further modifications.
Also there were not three Atlas-Agena pads at the Cape in 1963-64, there were two (LC-12 and 13). LC-14 was in the process of being converted from Mercury launches to GATV during this time and then it was used exclusively for those until being retired from use at the end of the Gemini program.