If someone asked "What is in the room?" and you responded "There's a table and chair," that's correct. Similarly, if you answered "There are a table and chair in the room" you wouldn't be wrong. In the first sentence, you're differentiating by type (there is one table and one chair/there is a table and there is a chair) and in the second, you're listing more than one object. Edwin mentions it in his comment, and here's a link so that you understand ellipsis, which is what allows you to say "there is a table and chair" instead of "there are a table and chair" if you wanted. The only problem is determining if there is ellipsis.
"There are" might sound wrong in that example because we recognize a table and chair as a set though. For example, people say "There is a knife, fork, and spoon missing from the table," instead of "There are a knife, fork, and spoon missing from the table." Neither of them is wrong, but when you think of the three utensils as a set, "is" suits the sentence better than "are". (A set of utensils is missing from the table.)
But if you were saying something like "There is a knife, fork, and nightstand in the bedroom," it sounds wrong (even though it isn't because of ellipsis) probably because we don't think of a knife, spoon, and nightstand as a set.
So if (like John says in his answer) structure and linearity as a set stifles creativity, then "the set (structure and linearity) is stifling creativity." I don't know why "what is" is in there, even if you didn't see them as a set, "structure and linearity are stifling creativity." The "what is" seems redundant. If it's in there, though, I don't see why it would change to "are"... it sounds like how teachers wanted you to answer questions when you were younger, where you use the question in the answer.
Q: What is stifling creativity?
A: Structure and linearity are what is stifling creativity.
But colloquially it seems like "There's" is used in place of "There are" and "There is" so... "There's" might be used in both cases.
In the case of which, if you can use "each of," "either of", "both of," "none of," and so forth in front of "which", you should probably just use "are". This is just going from personal experience, but I have always been corrected if I say "which is" and there's more than one object. I think ellipsis applies here too, but if I say "X and Y do this, which is Z," I usually get reprimanded and told to say "X and Y do this, which are Z." I was told to put each of/both of/none of in front of which and I'd see why that makes sense, but...
I have a knife and a spoon, which is made of silver. -> I have a knife and a spoon, each of which is made of silver.
I have a knife and a spoon, which are made of silver. -> I have a knife and a spoon, each of which are made of silver.
Both still seem right to me.