If you read carefully through the study, the lowest dose does not show anything close to maximum effect in several of the most interesting metrics they were testing (shrinking of white adipose tissue cells, serum levels of leptin indicating endocrine dysfunction). Basically they tested a ton of stuff, and actually selected a pretty good range to get a decent picture. If they wanted to zoom on on one of these, then maybe you are right they should have started at 0.1mg/kg... unless they were zooming on on say decrease in serum levels of leptins in response to benzene exposure, because no significant decrease would be found.
Also, they are obviously interested in lifetime exposure, but the expense of exposing rats to tiny amounts over the course of years is not financially/logistically feasible, unless someone is dumping obscene amounts of money into this specific avenue of research. Instead, they go with much higher doses in 24 exposures over a 4 week period.
Yes but "the dose makes the poison". Unless you're showing direct cumulative, irreparable damage, then it is not possible to extrapolate by poisoning something with a large amount of chemical that the small amount is just as bad.
Pure water - i.e. Milli-Q ultrapure water - is in fact a toxin in moderate quantities due to osmotic effects.
And 1mg/kg of body weight is way more than any person would be exposed to unless you like drinking gasoline.