No the buffer after pedalboard will stabilise your sound before it goes to desk - the pedals will see the same impedance regardless of what happens on the desk side.
I think the key to your problem is this:
the stage and monitor mix varies all the time for a variety of reasons e.g. different musicians playing at different volumes etc. but mostly as a result of hugely different stage volume for each session.
From that, I'd guess that you have monitors that change response at different volumes (or you're using different monitors in different shows). Plus of course, those old b'tards, Fletcher and Munson mean that we hear frequency response differently at different volume levels, which can screw with our perception of even the most carefully crafted tones. I'm also guessing you don't have a system where you have any control over your sound in the monitor mix without affecting the house mix? The engineer is EQing the guitar to sit in the house mix and that affects your tone in the monitor mix (which usually has a completely different balance).
BTW, if you are given a monitor with 15" bass driver, see if they can rather give you a 12" - 15s typically have a dip right in the vocal/guitar crucial midrange and are only really suitable IMO for monitoring bass and kick.
The solution is a dedicated powered monitor for your guitar. Something that you know and something that keeps the same (perceived) frequency response at varying levels and you can tweak quickly when needed. Feed the desk a signal that the engineer can do with what he wants and mirror/mult the same signal to your dedicated monitor. This keeps the guitar out of your normal monitor mix and gives you complete and reliable control over
your stage sound. Many modern powered monitor speakers have simple mixers in them so you can also use it for the monitor mix but still have control over the balance between the guitar and the other instruments and the EQ.
