I know it is unsafe to combine wires to avoid over loading a circuit; like
using two #14 to carry 18a. One could come loose and the other is
overloaded.
But how about doing it to avoid voltage drop?

I have no power at my dock, and carrying my generator down 130 stairs isn't
any fun. I have done it once, and probably never will again.
It is over 120ft, so a #12 extension cord would have excessive voltage drop
on a circular saw

A guy is selling a 250 roll of 12/4 wire for $50. As far as I can tell, it
is something for audio hookups and has no ground. Could I treat it as 12/3
with an insulated ground, connect two of the wires together to reduce
voltage drop, and leave it as a permanent extension cord? I figure that
with two wires connected, the VD would be reduced to a manageable level.
(yes, I realize the return is only single, but it seems okay if you average
the two) There is no reason the wires should ever come undone, but if they
did, the worst that could happen is burning out my saw.

Does this make sense?
Is this 12/4 stuff safe for 120v?

I did the same thing when I installed my transfer switch. All I needed was
12/2, but I used 10/3 in case someone needed it in the future; since the
labor was the same either way. I connected two wires together to reduce VD.
A little overkill, but it was just sitting there anyhow.