All of your sources refer to this study

For start, the authors did not find formaldehyde but formaldehyde hemiacetals. This is a combination of formaldehyde and alcohols (formaldehyde-propylene glycol or formaldehyde-glycerol). The authors characterized them as formaldehyde-releasing agents. There is absolutely no evidence that hemiacetals are toxic or carcinogenic. In fact, it is possible that the formation of hemiacetals might protect against damage induced by formaldehyde. Nevertheless, the authors considered the risk equal to formaldehyde and calculated the risk of cancer.

The authors fail to realize that voltage levels provide no information about the thermal load of an e-cigarette device. It seems that both the researchers and the reviewers who approved the study for publication missed that energy should be expressed in watts. As a result, we do not know how many watts were applied to the atomizer. However, there is a way to approximate this, through the information provided about liquid consumption per puff. The authors report that 5mg of liquid were consumed at 3.3 volts. Based on measurements I have performed, such consumption is observed at about 6-7 watts at 4-second puffs. Thus, the atomizer resistance is probably 1.6-1.8 Ohms. This means that at 5 volts the energy was around 14-16watts. That would be an extremely high value for most commercially-available atomizers (excluding some rebuildables which can withstand such high wattage levels). Thus, it is more than obvious that once again the atomizer was overheated, which of course will result in very high levels of formaldehyde production.