Sometimes things are exactly what they are and nothing more. I'm all in with the question everything and you are correct to do so. This is one of those that I think is just her signing I Love You and nothing more.

"investigation into sign language and its creator." Well, that's like saying who created any language. Primarily the people who use it. ASL is a combo of many other sign languages around the world and they all vary. Yes, I understand that languages are manipulated and we have to be careful about certain things and the "devil horn" gesture has ancient origins and therefor influence (on some).

This is a signing combo of 'I" "L" "Y" for I Love You. Not satanic. Besides that, as far as I know, the devil horn thing generally doesn't involve the thumb sticking out. It would be folded over the ring and middle fingers. I'm sure there are some cultures somewhere that do it this way. I really don't know. Point being, I think this is simply just what it is. I Love You.

Edit after rereading your comment: It is not traditionally a sign for satan either. It can be if that's what someone wants it to be I guess, but it is not universally. There is PLENTY of reading out there on it. That can be up to you to look into if you choose. I'm not going to summarize it here. It is not hidden info.