What is the interpretation of the parents' disapproval of the suitor, despite the istikhara and the mutual visions between the two parties that bring comfort, as if our Lord willed it?

Dreams have no connection to Istikhara. A dream of the prospective groom and bride seeing each other after Istikhara does not mean that Allah wills it. Rather, the parents' rejection may be an indication of a lack of goodness in the marriage. Allah facilitates marriage if it is good and turns it away if it is not.