Yes, it is possible to change the properties of the isl in memory without necessarily affecting the isl file itself. It's a bit tricky, though, and there's not a lot of documentation on it, since it's such an unusual thing to do.

If you want to change the resolution order for one particular control, (such as UltraButton) you could do something like this:

ApplicationStyleLibrary styleLibrary = new ApplicationStyleLibrary();
            styleLibrary.LoadFromStyleManager();

            StyleSetSettings styleSet = null;
            if (styleLibrary.StyleSets.Count > 0 && styleLibrary.DefaultStyleSet != null)
                styleSet = styleLibrary.StyleSets[styleLibrary.DefaultStyleSet];

            if (styleSet == null)
            {
                styleSet = styleLibrary.StyleSets.Add("Default");
                styleLibrary.DefaultStyleSet = styleSet.Key;
            }

            ComponentStyleSettings ultraButtonComponentRole = styleSet.ComponentStyles.GetComponentStyle("UltraButton", true);
            ultraButtonComponentRole.ButtonStyle = Infragistics.Win.UIElementButtonStyle.Flat;
            ultraButtonComponentRole.ResolutionOrder = ResolutionOrder.ControlOnly;
            

            styleLibrary.UpdateStyleManager();

To affect "All Components", the properties are directly on the styleSet object itself:

styleSet.ResolutionOrder = ResolutionOrder.ControlThenApplication;