Alex,
For item 1, I would use a DC-DC converter such as the LM2672 or equivalent for the 5V supply. Since it's a DC-DC type, it's more efficient (less heat and less battery drain compared to a 7805). It has a wide voltage input (made for automotive environment).
For item 2, I'll just use a 44k and 22k resistor voltage divider circuit, tap it from the middle for 4+ V when active high, add a 0.1 cap for filtering, a Schottky diode for backflow protection, and a 5.1V zener diode for overflow protection. All these will take up very little board space if you're using SMD components
For item 3, I'd use something like a VN750 or equivalent for high current requirement, or maybe a MIC2981 or equivalent to trigger some ~low power relays. It depend on the current requirement of the relay's coil. Look for this "highside" keyword when you're searching for this component.
(Maybe select a larger PIC with more I/O pins to do ICSP but the 12F683 may work.)
Good luck.