It’s possible (not always convenient) to enter Monaco from the Mediterranean, but getting into Andorra, San Marino, or Vatican City requires going through a Schengen country. Is it possible (and legal) to skirt the Schengen 90/180 rule by proving (for example) that thirty of your one hundred days were spent within one or more of the micro-states? How much/what kind of proof would be needed? Seems an entry/exit stamp would not be persuasive, since I could ask for a stamp, enter, and then promptly leave without a stamp.
For Andorra: if you have the corresponding stamps in your passport, the days there could be considered outside the Schengen Area.
For Monaco: You will recieve a French Schengen entry/exit stamps when coming in or travelling out through Monaco.
San Marino and the Vatican have open borders agreements, so days there will count as inside the Schengen Area.
In the end, the Schengen enter/exit stamps is what counts.
The Border control can, however, accept alternative proof should a stamp be missing. They, however, have the last say as to whether the proof is considered sufficient.