There isn't really a curriculum that both the selective school exam and Hendersons follow so those exams will still be accurate in some way or the other.
Idk dude, I doubt that doing the really old exams will help much with the newer exams, especially considering that the exams supposedly get harder every year. On top of that, many people stated that the last year exam was incredibly difficult for them and they had to guess the majority of the questions. I am unsure if this is a one/two off thing (last yr and yr before) but I reckon that the test will still remain at that level of difficulty to keep the pattern of harder and harder exams for each upcoming year. Don't get me wrong though, doing practice in any form is almost always good so using materials you have/can get is always a great option!
Also by the curriculum, I meant that the concepts in the sel exam are largely in the victorian year 8/9/maybe 10 domain, so I apologize if that was unclear

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Hope this helps!