"Of" and Relative Pronouns

No, they are not correct. The which-clause in examples 1 and 2 makes no sense as a nonrestrictive relative clause, so it should not be surrounded by commas. It is a restrictive relative clause. In addition, in example 2, the nonrestrictive (as indicated by commas) is placed inside the subject that it goes with, but nonrestrictive modifiers cannot be inside the NP they go with.

Because the subject is long and complicated, it is natural to put a comma after it, so the best version, imo, is:

The two strands of economic theory which are used in this article, are endogenous growth and spread-backwash effects