If it were self-reported information, people may lie about their intimacy. If it was tallying whether the children were ever married, intimacy was probably achieved and it seems valid. Internal validity is the extent to which the independent variable and ONLY the independent variable caused changes in the dependent variable. People chose whether to divorce or not. In some experiments, we do double-blind studies that eliminates the influence of demand characteristics and experimenter bias.
External validity is the extent to which the findings of the study are applicable to the real world.
The following is an experiment. I'd assess the degree of intimacy in the child's relationship in all 100 families before the manipulation. Highly unethical and immoral, but I need to manipulate divorce. Random assignment also removed any personality variables that might have explained the tendencies for divorced kids to lack intimacy. For example, in the quasi study described in Question 1, divorced parents might have argumentative personalities and these personalities were genetically based and parents passed on their tendency to argue to their offspring.