Bias-tee for adding multiple frequencies to a DC line
What you need is an inductor going into each frequency generator block.

Each of those block looks like a load for your frequency generator, the same as the 24V power supply does.

Everything that uses DC from the 24V line provides DC to the line needs an inductor.

Everything that puts a signal on the line needs a capacitor.




High frequency noise or distortion?  I expect you mean noise.  Twist the wires to reduce received noise.  You might also add ferrite beads to the lines, but I think the answer I gave above will get rid of most of it.
You are mixing the various signals on one line.  If you put two signals on the line, then you will see a mixture of the two.  This may be the "distortion" you are seeing.  You might also have problems with the different generators outputs being connected directly together.  In this case, a resistor in series with the coupling capacitors may help.
See above.  There's a very good chance that the power inputs to the frequency block short out the outputs when you get enough in parallel.  Again, an inductor in the power feed to each block will help.
You shouldn't need a termination resistor at the end of the string.  The frequency blocks put a load on the 24 volt line, so for the frequencies you are using you shouldn't need any kind of termination.




I think your inductors are too small.  At 8kHz, they have an impedance of only 23Ohm - that doesn't do much for you.  100mH inductors would have an impedance of 5kOhm, which would do much more for you in terms of keeping the signals away from the power consumers/providers.