You shouldn't have to spray BT the last few weeks before harvest. You can rely on it to be around for that long,unless you have serious rains. It's cool to mix it heavy,like 3-4 oz to gallon,it won't hurt the plants.
Just for future reference, if it rots. That's irrelevant to harvest time.Rot must be mitigated or acquiesced to and its timing has nothing to do with harvesting early.