To your point, it probably is a crappy attempt at overt sentimentality. After all, Moore was a straight-arrow MP in Vietnam until 1974, so what did he know? 1977 was his first year of dating, so he sounds like a nervous teen. He built up his dating confidence by 1980.
I'm not saying this is what happened, but I am trying to come up with plausible scenarios that rely on the fewest assumptions.
-PJ
How do you know it was his first year of dating? No one has suggested (I mean people who knew him then) that Moore wanted to date/court young women in their late teens because he lacked the emotional maturity to date women of his own age. The lack of emotional maturity argument has been advanced to explain some such relationships (mostly in the matter of sexual pederasty by priests) but I don’t think it fits here.
But even if he was emotionally mature that would not show through only when he wrote in a yearbook. But lets suppose it was an attempt to seduce Nelson. So he wrote an effusive, awkward, overly familiar dedication in an attempt to show his attraction to her. Was he hoping for a response? I have no idea. But it certainly does not follow that if he wrote that he would then force himself sexually upon Nelson.
What you are suggesting (I think) is that an emotionally immature man in his thirties wrote a yearbook dedication to a 17 year old girl in an attempt to seduce her rather a non-aggressive approach to establishing a relationship. Yet Moore also connived to take advantage of a vulnerable Nelson relying on her trust to sexually assault her and threaten her. He did this on the property of a place where they were both well known (according to Nelson). A bit of risky behavior on his part. Would an emotionally immature person who was not also a total sexual deviant take such a risk? Nope. I think I am safe in asserting Moore was not a sexual deviant.