As for the question of respiratory problems, did the researchers attempt to control for frequency of smoking, or past smoking? If not, then they are probably seeing the effect of a confounding variable.

Heavy smokers, long-term smokers, and smokers with trouble breathing are all more likely to try e-cigarettes as a substitute or a quitting aid, but to have trouble quitting completely. The same groups are likely more prone to experience a wheeze or cough, too. Taken together, this could explain why dual-users experience more frequent respiratory problems, but e-cigarettes are not the cause.

The study author admits this possibility, but assigns it equal likelihood as a novel (and IMO unlikely) interaction between e-cigarettes and regular smoking. She then emphasizes that e-cigarettes "cannot be marketed as a safe alternative" to smoking.

The author should have done the responsible thing and admitted that the study design does not allow her to make these kinds of statements.