Your answer for (iii) is a little lacking. The difference between (i) and (ii) is more than a notation change. What you need to exhibit is an isomorphism mapping S to S that "converts" the first relation to the second (and its inverse converts the second back to the first).
Your answer for (v) is true, but lacking in detail. For the two models to be isomorphic, you're going to have to produce an isomorphism that retains the relation. Why is this not possible between the integers and the reals?