No need to "melt" the BHO with a torch. Simply heat it as well and viscosity lowers enough to pour. I usually decarboxylate my BHO, monitoring temperature with an IR gun and the CO2 off-gas visually; And when I'm ready to mix, I simply pour in the oil. You can preheat the oil to 180-200F like you mentioned to dissolve faster, but with the BHO so hot and oil quickly heating, it should dissolve easily anyways.
Heating for 2 hours probably refers to decarboxylating the oil (making it orally active). SkunkPharm has a very descriptive page on it, but you can literally see enormous CO2 bubbles boil off, which is the decarboxylation process.
Some have questioned the available charts (like SkunkPharm's) regarding the degradation rate of THC to CBN. I know several visually fully decarboxylated oils not show nearly the amount of CBN as graphs would suggest, according to various lab test results. I don't know how quality the labs are.
Typically the decarboxylation graphs show the percent of THC as it rises to its peak at about 70% decarboxylation and then falls off rapidly.
The reason for the curve of course is after about 70% decarboxylation, the amount of THC degrading to CBN, exceeds the rate of the remaining THCa converting to THC.
Sooooooo, where is the CBN?
The answer is that CBN also degrades into its breakdown elements, and the total cannabinoids are lower.