1)
[実践的な] [防衛術の高度な技術] is also possible, the meaning is almost the same, though.
2)
Grammatically, the interpretation "polite form" is indeed possible, but actually it's said to Harry by McGonagall, i.e. from a teacher to her student, so "polite" is hardly used. In addition, I would use して差し上げます at the end of the sentece instead of してみせます to unify the level of politeness.
It's あなたが [必要とされる成績を] 絶対に取れるようにしてみせます(=闇祓いになるのに必要とされる成績をあなたが絶対に取れるようにしてみせます), thus, あなたが indicates the subject of 取れる, not 必要とされる, and される is passive.
This is exactly an example of "why context is so important", as in the linked thread.