T-ara members are tough. Have you seen their schedule before and after the scandal. Jiyeon and Hyomin have both tackled solo promotions (filiming dramas, movies and other projects) and T-ara at the same time. As insane as it sounds, I think they've gotten used to the work load. The only problem I have is that their weakness will be more glaring. I just hope it doesn't affect their confidence and that they try to strengthen themselves.
the ability to debut and be successful depends on how good your song is. Look at some of K-pop's best selling songs, some of them are done by idol groups that are not known for being vocalists (or having them even). Look at Gee, for example, it got popular not because of Taeyeon and her good singing abilities but because it was a fun, catchy song. People will like what they like. Don't talk about singing live like it matters all that much. T-ara isn't known for that so people won't have expectations. Besides, since the series of concerts last year, I think they've both gotten more stable.