I'd say Nos. 2 & 3, but there's a reason for that: those are the only two where the word preceding the verb ends with an r sound.
The contractions there're and where're are rather difficult to enunciate, whereas words like how're and what're flow much more naturally. Because of that, I think that you'd be most likely to hear Nos. 2 & 3, if you were to hear any of them at all.
Caveat 1: Of course, the verb in all 7 sentences should be are instead of is, but you already know that. I suppose any of them could be heard in speech somewhere – even native speakers inadvertently misconjugate every now and then, and some local dialects don't pay much attention to subject-verb agreement. But I think 2 & 3 are the ones you'd most likely stumble across in everyday conversation, for the reason I provided.
Interesting how you didn't ask about the opposite problem:
- How are your friend?
- There are a guy at the door.
- Where are my key?
- What are this cookie made of?
- Who are that person over there?
- Why are this student so bored with this topic?
I don't think I hear such questions uttered very often though.
Caveat 2 (added after some comments were made under my answer to the question): Just remember, an awkward, difficult-to-pronounce combination of words or syllables is not justification for changing the verb. "Here's your keys" is still grammatically wrong, even if "Here are your keys" might be more difficult to pronounce (especially if you've just chased someone for half a block, and you're panting and out-of-breath). However, the O.P.'s question asked, "Which are used in very informal English?" Fact is, grammatical rules are often disregarded in "very informal" English; ease-of-pronunciation is one of many possible reasons why, and the one I opted to focus on in my answer. There are other possible reasons, though, such as natural awkwardness, which is why so many people will say, "The data indicates..." instead of the more proper, "The data indicate..." – without an "s" at the end of data, it "feels" like the verb should agree with a singular noun.