Let's see if I can contribute a little bit, having been a Destroyer Fam decades before Harry Potter was born in JKRs brain.
First howevre you have one TERRIBLE sentence in this story:
"A sweep of Harry’s right foot twisted the next Death Eater in lines head so that he was looking backwards with a loud wet crack."
This would have been much better constructed with "...the head of the next Death Leater in line..." but if this is you only sin I can't complain (too much).
Anyway I can just see the meeting with Dumb-as-a-door after dinner.
"Excuse me you are all under the mistaken impression that I am here as an underage student, subject to this so-called Headmaster's whims."
(various sputtering from McG, etc, Filius smiling quitely, etc. Poppy reporting that the nothing, even skelegrow will repair Snape's broken bones for some reason, etc.)
"I am here on a business contract with a client, Professor Dumby, here being said client."
"I am a legal adult, and I am not subject to any of your whims or temper tantrums. As Snape found out, I will punish those without thinking twice."
"If I have any more crap from You Professor Dumby, I will invoke clause 32 of the standard Sinanju assassination services contract you signed."
"That is the "Treacherous Client" clause, which will cause us to be paid in full by the Goblins, after which I can assure you, my adopted Father will kill you, over a period of 30 to 40 days, depending on how strong your resources actually are."
"But considering how I already feel about Moldyshorts and his Death Eaters, I will probably wait until after I have killed him before reporting the invocation of Clause 32 to my Father."
"Maybe my Brother will decide it is his place to kill you. Remo likes things with sharp edges. He might decide to see if he can manage to slice 10,342 pieces off your body while you continue to live, going for a new record."