Startign with the easiest question, it's for the same reason that table salt is sold as 'table salt' and not 'sodium chloride', and that lye is not sold as 'sodium hydroxide'. It's just easier to remember and shorter, and has been called that for a long time. Look at other simple substances like lime, borax, flowers of sulfur, etc... just a name.

Sweet, I didn't know there was MgSO4 in it. I doubt it is at worrisome levels though. You flush anyway so even less worry. Obviously too much of any fert solution is potentially dangerous so go by the bottle dosage.

I have run sweet and molasses together just fine; you just use a lot less of each. I'd cut the dosage of BOTH. However I prefer to combine topmax and molasses to take advantage of the fact that topmax is a high concentration of carb synthesizers, without a lot of sugar filler.