2-way means backwards and forwards.
Not sure exactly what they mean by PLL mode (their manual has a code snippet for both modes, the snippets are identical). PWM is the standard pulse-width modulation used for speed control.
Greater external voltage will mean faster top speed yes - note the shield will swallow about 3V in the driver chip, so allow for that.
When is says 2A max, that's being extremely optimistic, it'll take it for short periods only, assume 1A as the practical upper limit for continuous operation - the driver chip will get extremely hot even then. The chip is available in an alternate package with a big tab for mounting on a heatsink - this is the version that can handle 2A properly (and even then it will lose about 5V which is a lot).