Questions regarding OFDM, FFT, IFFT
To your first question, yes, each symbol is sent on a separate subcarrier; that's the basic idea of OFDM.

I'm not sure where your confusion lies with your second question. It's well-known that the Fourier transform of a system's impulse response gives its frequency response. In this case, the system is the communication channel. If you choose the sample rate of your impulse response measurement properly, and use the appropriately-sized FFT, you can get a frequency response measurement whose bins line up with the subcarrier frequency grid. This can be used for equalizing the channel.