Fair enough points. Perhaps saying that there is more pressure would not necessarily be true, but the 8th inning situation is definitely the higher leverage situation. I'd want my best reliever out there.
As to your question about batting order, in what context do you mean?
If you mean how does it impact the scoring in these high leverage situations, the 'batting order doesn't matter much' relates to the course of an entire season. In any particular game, I think we'd all prefer to see our top hitters up in these situations over a struggling JBJ or Leon. That said, the difference in win expectancy of having JD up to bat with bases loaded in the 9th versus having JBJ up to bat in the same situation isn't that great. I don't know the exact % difference, but I know it isn't that big.