As for the other elements, there's a reason those are "missing". iOS already has UI elements that serve those purposes. Instead of checkboxes we have UISwitches. Instead of dropboxes, we have Action Sheets. For a good example, go to the Settings app on an iPhone and look through the settings; it's all form entry, but you won't find a checkbox or dropbox.
This is a conversation I've had with product managers and designers many times, and is part of the transition for people who are used to designing/developing for other platforms. For your app to look and feel like a native iOS app that users will intuitively interact with, it's best to stick to the system controls available. Or if you're making custom UIControls, to stick to the general behavior of the UIControl you're replacing. A checkbox on iOS would look fairly out of place.