Thank you for your praise for the plot. It's a great relief to hear you liked it, especially after I just finished getting rid of the biggest plot holes and hoping no obvious ones snuck past me.
The role reversal (Snape, not Harry, speaking Parseltongue) was an opportunity to try something new. The idea of Harry no longer being able to speak Parseltongue came from one of JKR's post-DH interviews, where she said that was one of Harry's abilities he had from Voldemort, and that after the events of DH he no longer had it. It was an intriguing challenge to write Snape as essentially a non-verbal character, since the verbal barbs and banter are so often the bedrock of his characterisation.
I'm glad that Ginny characterization worked for you. Thank you again for pointing out your favourite parts.