What is your degree of confidence in this conclusion?

In the world of scientific safety assessment, it is difficult to make any statement with 100% certainty. In this case, we have a substantial safety database that supports the conclusion that 1,3-D is unlikely to be a human carcinogen. The data from the oral studies provides some evidence of an increased incidence of benign but not malignant tumors. The studies conducted by the more relevant inhalation route provide even less evidence for a carcinogenic effect of 1,3-D.

There are enough studies to prove this conclusion.

Effect only seen in one study out of four available studies, the study may have applied doses above MTD, absence of genotox based on my reading of the gentox part, dosing above MTD; discrepancies in study outcome and kinetics in the oral studies.
If samples of the materials tested in the two oral studies is still available, it may help with data interpretation if the materials are characterized in detail regarding possible impurities