I don't like your examples except the your emphasis in example 2.
"Here's my fork. Where's your knife?"
I'm thinking hard and maybe the natural and native cadence is that I put the main stress on knife. If one puts an excessive emphasis on knife here, then I have no idea what it'd mean.
"Here's your fork. Where's your knife?"
New example. This is my example of knife emphasis. The possessive pronoun is the same but the object changes and the emphasis is there.
"Here's my fork. Where's your fork?"
Maybe I put default stress on the last word, but I really really have no idea what this means either if you put a lot of extra emphasis there. Why? Because the possessive pronoun is changing, but the emphasis is on a different word.
Much of the problem with the examples is the 1st sentence is framing the 2nd. And the emphasis word creates contrast with the first sentence. But you cannot change too much or then the emphasis makes no sense.
Ignoring the 1st sentence, because it creates a framing that makes several of these examples impossible:
- "Where's your fork?" The situation is a not-fork, and I want to clarify I am asking about the location of your fork(not a knife, not a spoon, not a bowl, but a fork).
- "Where's your fork?" The situation is about someone else's fork (or my fork), but we must discuss your fork.
- "Where is your fork?" Not where was, not where will, but where is it now?
- "Where's your fork?" The situation is discussing something about the fork, but not the location. The location is now the question. ("What's a fork?" "No... Where's a fork?")
- "Where's your fork?" This will not happen. The contraction will be expanded into 2 words instead.
If we start discussing emphasis on the question word, what I start thinking of is Abbot and Costello's "Who's on first?". A skit with a wordplay on player names and question words. There are Wikipedia entries in both English and German explaining the wordplay. The skit has them put emphasis on the question word in several examples. https://www.youtube.com/watch?v=kTcRRaXV-fg