Demux as a MUX

Am I understanding well that a mux can be used as a demux?

Please define what you mean by "mux" and "demux". It may be true (for instance) that some optical filtering components could be used for either purpose, but many multiplexing or demultiplexing devices permit signal flow in only one direction, and so cannot be used "backwards".

If the device is based on an analog switch, then it probably is reversible.