Maybe I can clarify some things...
Officialness: As it stands right now, no, we are not an official sequel, and we haven't claimed to be. It's a fan-game, unofficial, and that's that.
Future IP-related remarks: This is all stuff that's up in the air. The idea that we've talked about and would like to pursue if it's an option would be to do for Sierra games what TellTale has done for LucasArts games. As for the why us and not someone else....well, honestly, that question simply comes down to a question of ownership and permissions. If we, as opposed to any other group, were the ones to get either ownership of said IPs or permission to create official commercial games, then that's pretty much why us. But that could just as well be anyone else who had the interest, time, capability, and money to make such a thing come to pass. Blunt and un-romantic as it is, the question of "why you/him/her/them" comes down pretty much to money, drive, and opportunity. Why TellTale over anyone else? Because they're the ones who found a way to make it happen.
As for darkness/tone/etc: Personally, for the record, I loved the latter Harry Potter books.

I love that the series matured along with Harry and his friends, that they grew up and made hard choices, that they loved and lost and had to fight damn hard to win. Fairy tales that are simple and straight forward are nice, and they have their place too, but personally, I like something where it's not quite so easy.
But that's me, not everyone else. We liked the idea of taking the series to a somewhat darker and more mature place, instead of simply doing what had been done before. We wanted to take it that step further, to a place it hadn't really been before, and so we did. You may like it, you may not, and either opinion is fine. "Why?" in this case is, well, because that's we wanted to do.
I find the subject line of this thread interesting, and to me easy to answer. Why reinvent the wheel? The wheel gets you from one place to another, and it's basic design has been the same since it was indeed invented. But those first wheels? Wouldn't really be so good today. They'd be hell to use on cars, bikes, and other machines. They'd be uneven, rough-hewn, and made from very rudimentary materials. The wheel has been reinvented thousands of times over, if you think about it. All manner of revisions, improvements, adjustments to whatever it was specifically needed for, it's constantly being refined even today and it will continue to happen that way.
There are only so many stories out there, and they've all been told a thousand times. What's different is the way in which you tell the story, and this is, simply, how we chose to tell ours.