Most studies on pro-oxidative effects of E, C, CoQ10, ALA, Melatonin etc are done in vitro.
In vivo the effects don't seem to manifest in the expected way, hinting at the complexity of the issue. Context specificity seems to be crucial.
To take one example, consider E and C. E is assumed to be capable of turning pro-oxidative fairly easily, while C is mostly antioxidative except in particular in vitro circumstances. However when you combine the two, as would happen in vivo, you find that C appears to protect E from turning pro-oxidant by acting as a co-antioxidant, thus demonstrating a synergistic effect. See for instance:
http://www.ncbi.nlm.nih.gov/pubmed/10969031
For research on the idea that context is crucial, take a look at:
http://www.ncbi.nlm.nih.gov/pubmed/26236096
Or also:
http://www.ncbi.nlm.nih.gov/pubmed/25060102
As for cancer, most research is suggesting that since cells utilise oxidation to kill malignant cells, excessive antioxidants are logically a bad choice. But in acute circumscribed situations, such as meth binges, antioxidants are more than likely to have utility, as the overwhelming bulk of papers which have studied using them suggest.