I'll try to answer your questions in order.
That's because usually, i.e. in simpler transmitters, information is transmitted serially, i.e. one bit at a time. The simplest transmitter that comes to mind is an on/off transmitter: when you transmit the carrier this means one, when you don't transmit anything that's a zero. Even in slightly more complicated schemes, like QAM, you transmit 2 bits of data per symbol. Since your data is probably wider than 2 bits you need to split it in 'packages' accordingly to your modulation scheme.
In BPSK, i.e. bynary phase shift keying, the scheme is very simple: a one has phase zero, while a zero has phase \$\pi\$. Luckily enough this translates to a simple multiplication by 1 or -1. That's the coding, simple as that.
Apparently you forgot that 3 comes after 4.
Demodulating and decoding are two different things. In BPSK you first encode your data in \$\pm1\$, then you use it to modulate the carrier. Modulation in this case is achieved multiplying the carrier by the data, hence we are speaking of AM modulation. Since data is \$\pm1\$ we can also see this as phase modulation, i.e. carrier phase is shifted either by zero or \$\pi\$. When you receive the signal you need to demodulate it, bringing it back to baseband, multiplying it by the carrier, then you have to decode it, linking a +1 to a 1 and -1 to a 0. To do this you need a decoder that decides if the demodulated data corresponds to a 1 or a 0. I am not sure what you mean by hard decision, but again, demodulation and decoding are two different things.
Again, it seems to me that you are a bit confused about modulation and encoding. QAM is a modulation scheme that uses both the in phase and quadrature components of the carrier to carry data, so it allows to transmit twice the symbols. In a QAM modulator you can use two different encodings for the two transmitter sections, e.g. BPSK for the in phase branch and RTZ for the quadrature branch. When you receive the signal you first need to demodulate it with an appropriate demodulator that can retrieve both the in phase and quadrature signals, then each signal must be decoded appropriately, depending on the encoding scheme chosen for the transmitter.