1) the derailer doesn't care, that part is purely labeling aestethics and not functional. The cassette is harder to explain. Maybe they ran out of the correct parts at final assembly? Or human error. Or something.Or you missed counting a sprocket.
2) Shimano rear derailleurs are amazingly cross compatible and don't care much about what speeds they are listed at.
D-A may deviate, and I'm not too sure about the newer MTB-specific stuff.
But good ol' Deore shouldn't pose a problem.
Your Sora shouldn't be a problem either, as long as it can handle the tooth count difference.
The more speeds you run, the smaller the incremental change in spacing becomes from one to the other.
Half-conversions like that have been fairly common, with some people being entirely happy with the results, and others claiming that it's outright undoable.
The truth is probably somewhere in between, where certain combos of equipment conditions and riding styles does provide acceptable performance, while a more heavy handed rider, with gear that has just a little more slop in it will have nothing but frustration from it.
3) 8/9/10-speed all use the same size freehub body, and the cassettes all interchange, sometimes requiring a thin spacer to be added or removed. There's the occasional Dura-Ace that's an exception to the rule, but otherwise it's all mix n' match.
8/9/10-speed all use the OLD - Over Locknut Dimension(width between rear dropouts) of 130 mm for "road" bikes and 135 mm for MTBs. Hybrids can go either way.
But since you mention heavy gears I'm assuming you're planning to fit a wider range MTB cassette as well.
Personally, I like road-spaced cassettes, and would rather fit smaller chainrings. But it can be more expensive. And unless you're running a triple front, it is possible to feel limited by the available range offered by a small double front and a road cassette.