Here is my Gavin theory

Nigel is Gavin. Here is what I believe happened:

In the letter it says he was called “strange” pointing to the fact he was gay. The naked painting basically confirms it considering we know the artist was also a man.

I’m assuming that Nigel was in love with Gavin and (considering we know it was not socially acceptable to be gay during this time period) that Gavin accidentally killed him during a lovers quarrel or killed him when Nigel admitted he was gay.

Considering they were friends their entire lives, this would definitely be enough to send Gavin into madness considering he killed his best friend. He then assumed Nigels identity because of his guilt essentially killing himself and bringing Nigel back. Then he spends the rest of his crazy days searching for himself - Gavin.

The fact his model is named Gavin in the P.C. files also supports this.

I’m not sure what else could help confirm this. But I think there are probably more clues hidden in the game. Nigels body must be somewhere or at least evidence of his death.