1) What is the purpose of the light? work light or indicator? As alternate source could be used depending on purpose
2) You could keep both at 12v
3) ONLY use a diode on DC relays, it is for back emf suppression, on AC relays a resistance/capacitance snubber should be used.
4) The 15amp rating is probabally resistive load, when breaking a heavy DC inductive current, contact arcing and welding can occur, it is often better to break the AC side at the same time, use a multi-pole relay and disable the drives at the same time, if they have an enable input.
5)Depending on the VA of the transformer, I would expect 5amps max.
6) could'nt make that one out on the drawing
Note: Relays that break a heavy DC inductive load are fitted with magnets close to the contacts, called arc blow-out relays.