Oh yes, Varnish - the endless topic....I'll keep it short, even though the subject fascinates me....
I use several different spirit varnish recipes depending on the goal at hand. Copal does toughen the finish up a bit, but how much? I cannot really say. It is way harder to dissolve in alcohol, so I know the guitar will suffer less if the finished guitar gets wine spilled on it at a juerga or something....
A pretty good finish for toughness is two-thirds seedlac, one-third copal. Throw in a little mastic too, especially if you live in a place that gets really dry, and you're bad at humidifying the guitar. The mastic helps keep spider-web cracks at bay.
One of the others on the forum mentioned using egg-whites with coffee or something added to age the look. I have tried this but found it does'nt work too well. Try using seedlac as the first coat. Once the darkness you want is achieved, do the remainder of the finish with super-blonde shellac, adding copal to the super-blonde if you want.
Don't get oil in the wood!! especially a non-drying oil like olive or mineral oil. It will show up in the wood, and possibly soften the finish.
Build a U.V. booth and use this to darken the wood if you want a reliable method for aging - and it is consistent. Careful though with cedar tops; they darken very quickly.
Non of the shellac finishes are super-tough. But they are the most beautiful finishes, are very easy to repair, and virtually non-toxic. They are certainly the best finishes for musical reason's too.
Have fun learning how to french polish. It is well worth the effort!!