Can I replace multiple series diodes with one zener diode?
Theoretically, you could use a Zener diode.

However, there are several practical reasons why you would NOT do so.

1) Low-voltage Zener diodes are very leaky.  The threshold knee is soft and not well defined.  You have to run a fair amount of current through them to reach the rated Zener voltage.

2) Low-voltage Zener diodes are hard to manufacture.  The lowest-voltage Zener diode that comes to mind is either 3.0V or 2.8V - (I don't recall which).

For a low-power circuit, a string of series diodes performs much better.