Switch Rating for 50W LEDS
If each switch is supplying one string, you would just need to size the switch for the individual string - 3.75A.  Just make sure you are using a switch with a DC current rating of 3.75 or more; going off an AC rating alone is risky.
Yes, ground to the battery and switch on the positive supply line is fine.
You would need a separate resistor for each string but we can't speak to its size without knowing about the battery voltage.  However, if you're using a battery, the voltage will vary as it discharges and you're trying to run a pretty high powered system so a simple (high power) resistor is probably not the best choice.