#1. sure, that should work.
#2. one resistor should be fine but you should check the maximum current output of your microcontrollers i/o pin (check the microcontrollers datasheet) and compair it to the current draw of your IR LEDs. (check the LEDs datasheet.) add up the current draw of all the LEDs you intend to drive from a single pin.
if the total current you want to draw from a pin is more than the microcontroller can supply you will need to drive the LEDs with a MOSFET.
try to find a MOSFET that is designed to be controlled from a TTL voltage (IE, 5Volts).
alternatively you could just controll each led using a separate I/O pin and switch them all on/off at the same time.
i really don't know how much more range multiple LEDs will give you.
keep us informed of your findings
something else you should check out is the brightness of the LEDs. the LED datasheet should give you a "brightness" value. buy the IR LED with the highest value.