I find that if I store the unused Danish oil in the same can upright, a skin of cured oil forms on top and makes it hard to use the remainder.
This is due to the air trapped in the container after opening. Even if the product doesn't initially it's nearly inevitable that skinning over will happen — unless you're very productive and use up the contents swiftly — as each time the can is opened more air gets inside (and a slightly larger volume each time too) which provides fresh oxygen for the curing to proceed.
I have taken to screwing the lid on tightly and inverting the can so that the skin forms near the base of the can and doesn't obstruct the flow. However the screw-cap tends to glue-up solid.
Bravo if you came up with this on your own. Upending the can like this is an old-timers' trick for storing opened paints and varnishes1 so that the skin form where it isn't so much of a problem, underneath the liquid component rather than on top once the can is right side up again.
As for the cap sticking, heating the lid carefully with a heat gun, or sitting it into a shallow container of boiling water, could be enough to unstick the lid. But even with this you may find you need something like a plumber's wrench to twist the cap off.
I've never been entirely happy with this trick for a couple of reasons. One being the tendency for the cap/lid to stick badly.
But it also makes it hard or impossible to stir the remaining product. Thorough stirring is vital to consistent results with paints and any reduced-sheen varnishes (i.e. anything other than full gloss) which have some a matting agent added. Matting agents are like pigments, they start life as a dry powder which sinks to the bottom of a liquid during storage.
Last but not least I've had an upended can leak on me, and the results were not pretty!
I believe it is a curing oil rather than a drying oil
Rather confusingly those two things mean the same thing. So-called drying oils are actually oils that cure. Obviously a better name for them would be curing oils, which you will sometimes see, but the term 'drying oil' comes down to us from an earlier time and it's firmly embedded in the jargon at this point.
Alternatively should I switch to shellac flakes and just make up the amount of finish I need for a single project?
While I would highly recommend use of shellac to just about any woodworker (great traditional finish, still relevant in the right context today) "Danish oil" and shellac are by no means equivalent finishes, so neither one can be considered a replacement for the other.
However, there is much to be said for your idea of making up finish as you go and that is one of the things I was going to lead on to....
Make your own as needed
As I've mentioned in numerous previous Answers, finishes similar to "Danish oil" are merely a blend of some kind of varnish with more oil, usually with some additional driers to speed the drying of the slower-curing oil component. These blends are then, usually, heavily thinned with more spirits.2
Once you know the above you might begin to have the inkling that you can make something very similar yourself. And you can. I've never bought "Danish oil" but I have used an oil/varnish blend to finish things many, many times in the past.
My usual formula is a 1:1 mix of boiled linseed oil and oil-based polyurethane, which I then thin as much or as little as I like — this is another of the chief advantages of making up blended finishes at home, you can tailor the end result. You might make different versions of the product to suit your individual preferences, the type of wood, the weather conditions3, or the tightness of your time window.
Make your life simpler, use wiping varnish instead
On some woods there is literally no difference in the look you achieve, in terms of enhancing the figure, AKA 'grain pop', while being simpler to produce and, most importantly IMO, far more protective.
If a test reveals the higher oil content pops the grain more then I use BLO first, switching to wiping varnish to complete the finishing4.
And obviously I'm going to recommend you make up the wiping varnish yourself as needed, seeing as it's just standard oil-based poly thinned with more solvent... why pay over the odds for the finish manufacturers to dilute the finish for you?
And again, making it up yourself in small batches you can tailor the final product to suit your needs/preferences.
1 Note this is for oil-based paints and varnishes, it has zero benefit with other finish types which don't cure via oxidation. And with waterbased finishes doing this can have disastrous consequences if rust begins to form on the lip of the can or the lid itself from even the most minor scratches incurred when prying the lid off.
2 Other finishes in this class are what are commonly sold as "Scandinavian oil", "Teak oil" and "Tung Oil Finish" (which usually contains no tung oil at all!) and there are others.
3 For example during cooler weather or when it's more humid you might make it thinner specifically to help ensure each coat dries more quickly, rather than achieving the same end by adding additional driers (which you may not have, or may not want to use as they're often heavy-metal compounds).
4 Again to emphasise, only oil first if testing reveals that straight oil enhances the figure to a noticeably greater extent (you'll discover that in many cases it does not) and if the project requires this grain enhancement.... there are times when you don't want the wood to shout "Look at me, I'm gorgeous", but instead whisper "I'm very pretty" :-)