Answer to Question 1
In the Kelvin-Helmholtz contraction, they will become hot and luminous enough to give off detectable red light, but not for long, so you should look for them in star forming regions. Also, more massive ones than 12 Jupiters can shine briefly by deuterium fusion, but as the supply of deuterium is very limited, that process too will not last long.
Answer to Question 2
C