Flax seed oil only contains significant amounts of Omega-3 alpha-linoleic acid (LNA), cod liver oil contains significant amounts of Omega-3 eicosopentanoic acid (EPA) and docosahexanoic acid (DHA), and thus they complement each other very well. In my family we take a tablespoon of flax plus a teaspoon of cod liver oil whenever we don't eat enough wild salmon, sardines or other fish, higher in omega-3. Our omega-6 (LA) requirements are met with walnut oil (which also contains some LNA) and meats (AA).
As you know cod liver oil also contains high levels of pure vitamins A and D, which support healthy joints, bones, skin, hair and teeth. Studies indicate that cod liver oil helps to maintain the body’s natural defenses and contributes to healthy eyes and vision (also blueberries). Vitamin D plays a major role with Calcium metabolism, regulating absorption, excretion and retention in the bone. If you suffer from osteophenia you probably understand this relationship.
You confuse Oxalic Acid and Phytates in your nut question. Nuts like almonds or filberts are high in Phytates, but also high in Calcium. Phytase is an enzyme found in foods high in Phytates (also whole grains, seeds and legumes). You can activate Phytase by gently heating or sprouting. This will allow Phytase to break down Phytates. If you have read my past messages you know that I am strongly in favor of fermenting, sprouting and gently heating (gently heating e.g. flax seeds to also remove the possibility of cyanogenic glycosides, a form of cyanide, which can be found in some seed and nut varieties).
I can see that you also understand the relationship between Calcium and Magnesium, very important.
If you consume Calcium from a number of sources (cheese, broth, mineral water, black strap molasses, yogurt, rice drinks, cheese, soaked nuts, sesame seeds, fermented legumes and dark leafy greens) like my family does, you don't need to be concerned about dietary interactions, especially leafy greens.
Just understand the metabolic response to calcium, especially if you are older or over 60 years old and suffer from stomach acid lack (in which case you might need solubilized Calcium). Understand that sugar, alcohol, caffeine, lack of exercise and a high protein diet increase urinary Calcium excretion.
Avoid dark leafy greens in off seasons, e.g. never eat rhubarb after April/Mai, same for spinach. Oxalic acid content increases dramatically during the warmer season. Beets, New Zealand spinach, parsley and salads are of course ok in summer. Eat collards and kale only in late autumn, early winter (late winter if they are grown further south).
Hope this helps you to make more informed choices.