Parboiling potatoes to make them softer in a stew
45 minutes of boiling is more than enough time to soften potatoes regardless of altitude.

Parboiling potatoes before roasting frees up and gelatinizes starches on the outside of the potatoes that then get nice and crunchy when roasted. It will not do anything useful in your stew.

The likely culprit for your potatoes not softening is probably acid.

When potatoes are boiled with acid (lemon juice, vinegar, tomatoes, etc) the acid and heat will cause the pectin in the surface of the potatoes to gel. This is sometimes desirable for things like French fries but it can make them too firm in other cases.

If your recipe has acidic ingredients you can add them near the end of cooking or at least after the potatoes have had time to soften properly.