Numbers stations are meant to be cheap and dirty, at least as far as
the folks in the field are concerned. So what numbers station is going
through such hassles to do some multiplexed transmission when there
isn't exactly a high premium on their broadcast time. That is, if they
want to broadcast more signals, there is plenty of time to do so.

I'm guessing by angle modulation you mean phase modulation, and SSB is
not phase modulation.

If you understand mixers, you generate AM by feeding an audio signal
that is riding on a bit of DC into a mixer. The DC into the mixer
produces the carrier, and the AM into the mixer produces the sidebands.
Without the presence of DC, you would get DSB (double side band),
which is like AM with the carrier suppressed. To generate SSB, you
could either filter off one of the sidebands of DSB, or uses two mixers
in quadrature and feed the audio through a Hilbert transformer to get
it into quadrature, then sum the right signals to get either LSB or
USB. The diagram should be in any basic communications book and
probably on the net if you do some searching. The Hilbert transformer
is a mathematical notion, so the book might just show a box that takes
0 degrees and outputs 90 degrees independent of frequency. In real
life, how this is done depends on the relative bandwidth over which you
want to achieve the 90 degree phase shift. Often the audio goes into
two filters designed such that the difference between these filters is
90 degrees. Modern writings on Hilbert transformers are likely to be
DSP, so you would need to find a book (i.e. pre-Geek) to see this done
with analog filters.

I'm going to pass on the demod question as you phrased it, but I think
if I had to recover some stealthy USB riding on top of AM, I would
recover the LSB of the AM signal as one signal. Then recover both USB
signals (which are really sharing the same spectrum) as another signal,
then subtract the demod LSB from the demod USB combo to yield the
stealthy USB audio.