Question 1 :
The three modulation schemes you mention are all forms of digital modulation. As you allude to, information can be encode in the amplitude, frequency, or phase of a signal (frequency is the time derivative of phase). For digital communication in general:
- ASK is very inefficient and susceptible to interference for wireless (but can be useful over fiber optics or copper).
- FSK is less susceptible to errors and interference than ASK because interference is often confined to a specific frequency.
- PSK is the least susceptible to error of these three. In terms of bit error rate (BER), binary PSK is about twice as good as ASK and FSK in the presence of noise, and the BER can be significantly improved through other techniques.
QPSK has the very significant advantage that it can be used to double the data rate over the same bandwidth, or half the bandwidth for the same data rate. Further, as the number of signal states increases from 4 (quadrature) to N-ary, the trade-off can be furthered. As N increases the complexity of the receiver increases.
You will want to use a combination of FEC and re-transmission. FEC, or at the least error detection, should be used to know if a packet of data has been received correctly. Re-transmission WILL be required because it is inevitable that data is lost at some point, so the system should be able to request a packet be resent if it is incorrectly received.