This tends to confirm my own feeling that Harry and Snape are themselves very alike

I would fully agree on that point, and in that respect it is rather nice to have this literary precedent who can somehow highlight their similar origins as character types. If I ever manage to unearth myself from the heap of RL essays and commissions sitting on my desk these days, I really should wrap up my analysis of Harry/Snape/Voldemort's triple-embodiment of this character archetype. As it is, I'd say it's rather a good thing that Dominic retains his own traits, as it does keep Harry and Snape's own distinctive qualties firmly in mind - one doesn't want to cast too broad a sweep over the lot of them.