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David, I believe you correctly recalled Carlson's outline of his use of copal. Simply put, good mediums are a balance of solvent, (to control viscosity and drying) a resin (for the character it imparts to paint) and oil (plasticizer and binder).
Zachary, for re-touch varnish, damar or copal are the obvious best choices. Starting with a saturated solution, (usually a 5# cut) either should then be reduced 50% with turps for retouching. Either will serve the purpose of "pulling up" sunk-in passages and visually unifying the surface of the painting. Damar remains re-soluble in turps, copal does not, and that makes it an ideal isolating varnish for working further.
Note the small amount of solids in retouch varnishes is designed to result in application of a very minimal amount of resin to the surface. While "overcoating" with a painting medium (whatever it may be) would certainly result in "raising" the sunk-in pasages and unifying the surface, that's inadvisable because too much "fat" material would be incorporated into the uppermost layer. Keep in mind in time, all vehicles (oils, resins, solvents) ultimately yellow and darken, and the best way to prevent that is to maintain an optimal balance of pigment and vehicle in paint layers.
"Oiling out" involves working a very small amount of linseed oil into the surface of a thoroughly dry painting with the heel of the palm of your hand using enough friction to warm the surface. The purpose is to "balance" the oil content of lean, sunk-in areas. By definition, linseed oil is a "varnish", and oiling out balances the oil content of lean passages, and does unify the surface. (and without a lot of gloss)
Problems varnishing commission pieces? Unfortunately for me, my work hasn't been distributed widely enough (yet!) that re-visiting a piece to apply a coat of final varnish hasn't been a problem. It's not a bad thing to have this periodic contact with your client, and a makes a good excuse for keeping in touch. It's good PR too, demonstrating your comittment and reliability. As for waiting, it's usually possible to apply final varnish well inside of a year.
If you desire a "flat" final varnish, a mixture of damar and high quality beeswax will produce a reasonable varnish that pretty well eliminates sheen. The drawback is that the "real" value of dark passages is compromised, since a "flat" surface presents what you're trying to avoid by retouching a picture . . . that is, it looks "sunk in".
I would avoid applying wax only, because most wax preparations are not hard enough to be cleanable, and the surface of that film will more readily absorb dust, dirt and soot.
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