I'm not entirely clear on that either, but it must have something to do with how both of their wands contain one of Fawkes' feathers.

I wondered too. But if you compare Harry's "interrogation" (if you can call it that when Crouch talks more than Harry does) to Crouch's interrogation a bit later, it seems obvious Harry wasn't under the effects of Veritaserum. He is too reluctant, too much focusing on other things than the questions asked, and not talkative enough.

Yes, that would be very useful indeed. Just think of all the chocolate you could keep stored in that place where Moody was imprisoned.