Per Wikipedia, an
imaginary number is: "... a number that gives a negative result when squared..."
I'm quite comfortable using the term "imaginary" in the context of the math... but I'm not aware of current, voltage or magnetic flux being described as imaginary.
I'm also comfortable with the idea that many wave fronts can arrive at an antenna though there is only one source. There can be many paths of various lengths and may have fixed or variable attenuation in addition to still other signal effecting factors.
My understanding is that all of the magnetic flux, from all sources, impinging on a conductor at one moment, may be summed algebraically to then determine the change vs time that will induce current in the conductor. Even though many magnetic fields may be influencing the conductor, only one current is induced. This would be reasonable given that energy has changed state, from a magnetic field to an electrical current.
I see multipath beginning at the transmitter and ending at the antenna, not the junction of the antenna and transmission line or later in the tuner. But yes, the tuner will have to deal with the effects of multipath because it will have possibly modified every measurable attribute of the signal. Multipath can be viewed as an interfering source of modulation. It will obviously alter the amplitude. Doppler shift can result from reflection off of moving objects, and phase shift would result as well. Perhaps CW would be the least likely modulation.
Yes the gears are slowly squeaking in this old head...
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the impedance of the antenna does vary by frequency, but for a single frequency it remains the same no matter the direction the signal comes from.

Agreed. And the impedance will not change if the amplitude of a given signal varies.
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My view of what is happening is the multiplicity of signals out of phase with each other march down the coax until they hit a detector.

I believe fourier analysis gives you a mathematical tool set to analyze from that view... though in a real series circuit, the current is equal at all points, (phase not withstanding).