Pilot symbols in IEEE 802.11a
A frequency offset $f_\mathrm{off}$ between the carrier frequency and the local oscillator (at the receiver) results in a linear phase, i.e. the received signal contains a factor $\mathrm{exp}(j2\pi f_\mathrm{off}/f_\mathrm{s}n)$, where $f_\mathrm{s}$ is the sampling frequency and $n$ is the discrete time. If the frequency offset is sufficiently low (e.g. because it has been estimated and compensated for in an earlier step) the linearly changing phase can be assumed constant during one OFDM symbol. This means that all samples of the received OFDM symbol have a constant phase offset that can be estimated using the pilot subcarriers. As the phase is changing slowly, this has to be done for every OFDM symbol individually. The pilot symbols can also be used to estimate phase noise.
Yes, see 1. Frequency and phase offsets are time varying effects while the channel is assumed to be constant for at least some OFDM symbols.
Yes, pilot tones are sent in every OFDM symbol, along with data subcarriers.