IOS – I loaded iOS7.0.2 on the iPhone and all of the purchased apps I had have disappeared from the screen

When you purchase apps (even free) on the device, iTunes always backs this up - and when it Syncs, before it does anything it backs up - and it notices that you have purchased an app/apps - so it wishes to make a backup of these apps. Click transfer and it'll all work fine - happens to me when I buy songs off of the store as well as apps

Apple has worked hard to try to avoid the situation you mentioned. It is a major pain to have apps downloaded from different Apple IDs on one iOS device. For example, someone bought an app for me, but used (obviously) their Apple ID. Anytime I wanted to update that app, they had to enter their Apple ID in. That also meant that clicking "Update all" in the app store didn't really work. I finally got frustrated and bought the app myself.

So the short answer to both of your questions is basically yes. You can download it for a friend, but, anytime they update, you must autenticate for that app. Everything else should work normally.

You mentioned iPhone vs. iPad - as I'm sure you're already aware of, note that the app must be compatible for iPhone and iPad for what you mentioned to work.