1. Yes, true, I did say that Jirachi has low offensive presence, but U-Turn is not for attacking purposes. The damage from U-Turn does add to LO recoil and other residual damage, which is a plus, but the point of U-Turn is to slowly switch out to a scarfed / priority user to revenge kill Mewtwo. Seeing as Mewtwo can't take down Jirachi in most circumstances, and Mewtwo is naturally faster than Jirachi, it is almost guaranteed that the revenge killer you wish to use will come in relatively unscathed. This combined with possible Wish support makes it usesful.
2. I'm not sure just how relevant Mewtwo vs. Jirachi is to the CCAT, but seeing as we have no control over the weather, Jirachi is at risk of being 2HKO'd in neutral weather or OHKO'd in Sun. That's probably one of the reasons we didn't pick Jirachi. However, outside of the CCAT, Jirachi is one of the better checks/counters (which one is it?) to Mewtwo seeing as it will be able to do the things mentioned above as well and have rain support as well.