Xautoconfig probes the current monitor mode and creates a default config file. I don't know what 15 actually is, but it implies 5 bits for each primary color, and that's the one that some monitors want. Xautoconfig will likely create multiple Display subsections, each of which has multiple modes. Xconfigurator creates many lines of mode information that are either irrelevant or unneeded for a particular monitor. Xautoconfig adds some default modes and the currernt console video mode.
X depends on the correct keyboard and mouse drivers.
The choice of keyboard settings shouldn't prevent X from starting, but will determine whether the keyboard map is correct. Run kbdconfig before Xautoconfig4 . The most important key to understanding X 4.x is the video card device. Switching to Xpmac may eliminate this.