Yes, the WC question mark is an interesting point to consider. It seems like almost a foregone conclusion that we will be one of the WC teams, although acquiring an elite pitcher at the trade deadline would still give us the chance to make an extended run during the season and possibly catch up. I wouldn't rule out the division in terms of the goal at the deadline if we have gotten to within 2-3 games of the Cards by then, but that's a big if.
Cueto would definitely be an interesting target, although I agree that it wouldn't be worth sending good prospects to a division rival and you have to imagine he'll be one of the best available players. I don't know why the Brewers would look to deal Fiers, and I'm not really that high on him, so I wouldn't spend any good assets on him.
One potential rotation boost in August is definitely Taillon. It won't exactly be ideal in terms of the pressure of a playoff race, and it also depends on injuries, etc., but Cole did something similar, and replacing the #5 starter is a lot less pressure if 1-4 are still performing then how they are now.
That boost would be free, and simultaneously allows us to see what we might have as part of a 2016 rotation. I can't imagine Taillon will need that long in AAA, and the upside with him is a lot more than looking for an average #4/5 type to replace Locke.