The .dropdown-backdrop element isn't used on iOS in the nav because of the complexity of z-indexing. Thus, to close dropdowns in navbars, you must directly click the dropdown element (or any other element which will fire a click event in iOS).
Page zooming inevitably presents rendering artifacts in some components, both in Bootstrap and the rest of the web. Depending on the issue, we may be able to fix it (search first and then open an issue if need be). However, we tend to ignore these as they often have no direct solution other than hacky workarounds.