Q&A (Seriously, this is not a chapter. Read the previous entry (i.e. the finale) for this to make sense.)
Why an alicorn? – A simple complexity. There was a thought process here. 1) Alicorns aren't inherently special. 2) Huh… Alicorns used Dark Magic pretty often… 3) Alicorns are often seen to represent the harmony of the pony types… maybe it can work for magic as well. 4) Alicorn Grimoire!
Why can Grimoire switch between forms while it seems the princesses cannot? – He explains it, actually. The alicorn princesses were either born or granted their powers (depending on what you choose to believe). However, in this from (of dubious canon) those that gained the power were granted the power through one magics rather than the combination. Grimoire mastered both Light and Dark, so he can use the combination to switch between them.
Why doesn't Grimoire just stay an alicorn? – Because it would be boring. He knows that alicorns are seen as special and regal. He doesn't want that. He wants to walk around and observe without ponies staring and thinking he is royal. He's a field worker by heart. He wants to remain unimpeded and open with his experimentation. He finds the alicorn form to be too inconvenient.
Wasn't Grimoire the villain? – You've activated my trap card! Grimoire was written to appear as a villain when you didn't have all the pieces. His use of Dark Magic is meant to play on expectations. This story has no villain, just a pony with a different outlook.
Twilight was out of character when she accused Grimoire! – Was she? Consider this: you grow up in a society that teaches you that Dark Magic is the magic of hatred and you've only seen and heard about the evil. Would you be so quick to trust someone with that magic?
So… a unicorn that can use Dark Magic can become an alicorn? – Not necessarily. As established, unicorns need to be able to combine the magics on a deeper level. Consider the symbol of Yin and Yang. Though the two halves are balanced and even contain pieces of the other, they remain separated to black and white. Grimoire, through study and a hint of luck for the first transformation, was able to break those halves down and turn them grey. This is a difficult process as it means you need to break down everything you understand and bring it together.
What is Dark Magic to you? – Magic in this reminds me of the Force from Star Wars. We see the Sith as the bad guys; but, if you break down their actual philosophy, it makes things seem different. They use the force rather than act in the wishes of the Force. It's a difference between active and passive life styles. Being a Sith doesn't make you evil in the same way Dark Magic doesn't make you a villain, it's what you do with it.
How many journals are there? – 11 (A through K) have been completed, and 2 (L and M) are being worked on as we speak. He uses the first letter to keep track and the rest of the call number to be more memorable. A-Z-26 (a reference to the Latin alphabet), D-Q-17 (Dairy Queen? I don't know. It just stuck.), K-M-11 (if you don't know where I got this one, I can't help you.), and M-A-50 (the Roman numeral for 50 is L. I'll leave you with that.).