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All of which points to the dismal state in which we find ourselves in the 21st century, with such uncertainties in obtaining a working knowledge of oil paint, its nature, and the relatively simple materials, principles and processes that comprise sound craftsmanship in applying it. Spurgeon's site is a good source of information, thank you for noting additional sources.
Back to the initial question, in general, a painting 8 to 12 months old may be considered to be "thoroughly dry", i.e., all free solvents will have evaporated, and all paint films are firmly "set" into irreversable polymers. Of course, variables obtain, such as the thickness of the paint, the materials used, and ambient conditions.
To continue painting on an aged surface, most painters will require to properly assess the true color and value of dry, sunk-in passages. "Fat" areas that present a slick, shiny, or even tactilely oily surface will be problematic for adhesion of subsequent applications. Both problems are best solved with a sparing application of retouch varnish.
Damar will remain soluble in the presence of solvents used in overpainting, copal will not. Either may be more or less agreeable depending on one's method of working and the end in view. In short, there's nothing at all wrong with going back into a painting that has been "stalled" for months . . . or even years. In fact, it is better practise to paint over a thorougly dry surface than it is to begin painting over surface-dry underlayers, where overpainting can interfere with proper curing of the first layers of paint.
"Oiling out" is a process with limited application, and even less appeal, and no one really knows how to do it, or when to apply it. In the main, linseed oil (and other drying oils) have been considered "varnishes" from time immemorial. When a completed, thoroughly dry painting presents dry, sunk-in passages, no more than a drop or two of linseed oil, applied to those surfaces, and rubbed in with the heel of one's palm with enough vigor to generate warmth from friction will correct the problem. It is no less than miraculous how far a drop of oil goes in this process. The result is that the lean passages are thus "varnished". Applying a 50/50 mix of oil and solvent with a brush or daub is a different thing entirely, and invariably deposits enough oil to be deleterious to the painting as it ages.
Since the question is actually how best to unify the surface of a completed, thoroughly dry picture, application of an appropriate final varnish is the best solution, since it's really quite doubtful those "sunk in" passages actually display the results of paint so underbound that an additional application of free oil is called for.
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