Greetings 11th corp.
Watco Danish Oil finish is an old, old furniture finish that is produced in at least four types: natural, light walnut, medium walnut, and dark walnut finish.
I have used it for numerous applications on various woods almost from its inception, and have always had good results.
As you have discovered, it does a nice job of staining maple and is virtually mistake free in use. This feature was particularly nice when I taught a Middle School Wood Working Class for a couple of years.
Oil finishes enhance and bring out the figure of wood better than any other type of finish, and Watco is no exception, however, Watco is not a good exterior finish.
You mention finishing off with polye-urethane varnish. You might want to consider instead, finishing off with a high grade of tung oil which is very compatible with Danish Oil and will seal over the Danish.
Another good, compatible oil finish over the Danish Oil is BIRCHWOOD CASEY Tru-Oil Gun Stock Finish. It is available in a rub-on or spray-on aersol can. Tung oil can always be applied over the Tru-Oil for touch-up too.
DEM-BART Continental Style Stock & Checkering Oil is also good for sealing over the Danish Oil, but is somewhat more difficult to use.
One of my all time favorites is Flecto Varathane Penetrating Plastic Oil Sealer. This is a finish recomended by the late John Bivins as an all purpose complete finish.
Unfortunately, it is no longer available in my area or even in the surrounding area. If you see this product anywhere, grab onto it; you will not regret it.
There is nothing wrong with your plan to top off with a good varnish. Varnish was around 200 plus years ago, but most gunsmiths or cabinet makers had to make it. "Secret Formulas" really were kept secret in those days. A word of caution though; not all varnishes are compatible over oil.
I have been finishing furnitue and stocks for 50 plus years and like John Bivins, keep looking for the "perfect stock finish". This quest is the the Holy Grail pursuit of stock makers.It never ends.
For the past 20 years or so, my perference has been to use compatible oil finishes for my stock work. That has simplified that part of my life.
Hope this information will prove useful.
Best regards,
John L. Hinnant