It took large scale randomized studies to establish the negative health effects of vitamin supplements/antioxidants and HRT both of which appeared safe in the short term, exploited a regular physiological process and had a long track record of use. I'd want to see a large randomized study of the long term effects of melatonin use in adults to establish the long term effects of melatonin use in adults.

The Wikipedia link you give merely concludes that "evidence suggests that melatonin is safe with short-term use, three months or less". From your other links:

The findings of this review suggest that exogenous melatonin is a relatively safe substance when used in the short term, over a period of days or weeks, and is safe at relatively high doses and in various formulations. However, the safety of exogenous melatonin when used in the long-term, over months and years, remains unclear.

There are no published long-term safety data on the use of melatonin for whatever purpose, assuming long term to mean more than 6 months of daily medication. In the light of its physiological role in animals, the potential deleterious effects include inhibition of reproductive function, delayed timing of puberty, and influence (when taken during pregnancy and lactation) on the circadian status of the fetus and neonate and on future development. Its interactions with other medications are virtually unexplored. For most positive effects published, there also exist negative reports.

There is evidence to suggest that melatonin is not effective in treating most primary sleep disorders with short-term use (4 weeks or less); however, additional large-scale RCTs are needed before firm conclusions can be drawn. There is some evidence to suggest that melatonin is effective in treating delayed sleep phase syndrome with short-term use. There is evidence to suggest that melatonin is safe with short-term use (3 months or less).

The evidence you present for the benefits of melatonin is also weak and the Wikipedia article is fairly circumspect about the benefits of melatonin. The first review you link to states:

Melatonin decreased sleep onset latency (SOL) in normal sleepers (weighted mean difference (WMD): -3.9 min; 95- percent CI: -5.3 min., -2.6 min.). The magnitude of this effect appears to be clinically insignificant. There was evidence of possible publication bias in the selection of studies that were analyzed; we found a greater number of studies reporting positive results compared to negative results.

and

Melatonin increased sleep efficiency in normal sleepers (WMD: 2.3 percent; 95-percent CI: 0.7 percent, 3.9 percent), and this effect was dependent on the timing of sleep, such that the effect of melatonin was greater in daytime sleepers (daytime sleep: WMD: 8.0 percent; 95- percent CI: 1.0 percent, 15.0 percent; night-time sleep: WMD: 1.2 percent; 95-percent CI: 0 percent, 2.4 percent). The magnitude of this effect appears to be clinically insignificant. There was considerable evidence of possible publication bias in the selection of studies analyzed; we found a greater number of studies reporting positive results compared to negative results.

and

Generally, these studies were of low-to-moderate quality.

Nowhere have in these links do I see any evidence supporting your key claim that melatonin allows for the benefits of 8 hours of sleep in 7 hours.

I think your post spent too much time discussing the relatively uninteresting topic of the cost of melatonin and not enough on the evidence for safety or efficacy. Based on the discussion and the further research you and others have linked I'm not persuaded enough by the evidence for safety or efficacy to adopt melatonin for long term use, but I may try it out for jet lag or other short term sleep difficulties.

I believe it's not generally considered valid to apply results from medical studies on adults to children. I'm not sure if the reverse applies.