Can the number given to the Cellular version of the Apple Watch be reached over the PSTN? From previous experience, my understanding is that if a Cellular Apple Watch is purchased and then a plan added to the Watch from a supporting carrier, the watch is given its own mobile number. Can this number be reached via the PSTN, or is it only an identifier that the Watch uses internally and not accessible from the outside?
Are you perhaps confusing an eSIM with a telco technology PSTN?


An eSIM is an Electronic Subscriber Identity Module.  It's the digital version of the "old fashioned" SIM or the plastic card that you put into the phone to get onto the cellular network. It's not tied to a phone number per se.  It's tied to an account that allows you access to the cellular network.  
The PSTN is the Public Switched Telephone Network. It's the aggregate of all the switched circuit telco systems including cellular.
A phone number is just a sequence of digits assigned to an account so that another telco device can connect to it.  It's just a "pointer" to an account - you dial the digits and the phone company can make a single phone ring or multiple phones around the planet ring simultaneously (and a number of variations in between).


As far as how the iWatch's eSIM works, it is attached to your cellular account and for all intents and purposes, shares your existing phone number.  It's not given it's own unique phone number.

It's important to note that an eSIM or plain SIM do not have to have a phone number - any mobile hotspot that you can't make calls to won't have a phone number as the plans for these devices are data only.