Steve- you fundamentally misunderstand the difference in the media. Varnishes over oil are
designed to be removed and replaced if necessary without destroying the painting. Try putting the same thing over a watercolor and you've inherently ruined it forever. Typical
photo varnishes were engineered to supply a retouching surface, not for archival properties. They yellow in time and eventually cross-link, get brittle and crack. In Strand's
time this would have probably been something akin to a wood varnish or perhaps nitroc.
lacquer - solvent might work, might not, with bits of lacquer chipping off and taking emulsion with it. I've been involved as a consultant on quite a few analogous restoration
issues architecturally, and someone's mistake decades ago can equate to hell today.