I add all my sugar additions at the end of the boil, unless I forget.
When I forget, which happens, I just add the sugar straight into the carboy. Usually this is table sugar, not syrup. Once the fermentation is going, I think the yeast will find the sugars so I'm not too worried about whether or not they settle to the bottom. I don't think I'd do a whole 1.5lbs at once though. I'd stage that over a couple additions.
I don't see any impact in attenuation in adding everything in the kettle. I don't have a comparison, but my attenuation tends to be very good and I would not want it to go further.
As far as sanitation, the brix of sugar/syrup/honey should be high enough that nothing's going to grow on there. I wouldn't worry about it, and I don't.