Maximum number of inversions that can be removed by swapping two elements?
The question is indeed not completely clear, but they definitely wanted you to search the "worst possible" array $a$ (the one leading to the largest number of removed inversions). Otherwise there is nothing to do - just look at the array and calculate the number of inversions resulting from the swap.

What is somewhat less clear is whether they also wanted you to optimize over $i$ and $j$. Your answer, which doesn't assume that, is on the safe side.

One thing you haven't shown is that your example leads to the maximal number of inversions being removed. The argument is very simple, but it's still needed for completeness.