am i going crazy?

You're crazy. The music is composed and performed by Eddie. When it comes to music - while Sammy was with Van Halen - Eddie was in charge. Sammy wrote lyrics an Eddie signed off on the melody Sammy sang/screamed. Eddie is a control freak in the studio which is why tensions run high between Eddie and lead singers.

Furthermore, Eric Stolz has only a bit part in "Fast Times..." He (and 'Goose' himself) are Spicoli's stoner buds at the burger joint. That may be Bill Conrad but given Bill's relative Wild Life virginity it's doubtful that he emerged from Spicoli's sphere of influence.

Sammy Hagar did the theme song for 'Fast Times at Ridgemont High'. Maybe you got the two of them confused.

The day Chuck Norris dies is the day the world ends.

You're definitely not going crazy. Eddie Van Halen was still with DLR at the time, but I'm sure things weren't going well. The music you hear on the soundtrack features some riffs from some future songs. This is pretty common and happens a lot when a famous person does a soundtrack. Another example? The movie Night Shift features an early instrumental version of "That's What Friends Are For" because Burt Bacharach did the music.

As for this being a sequel, it is, or it was. It was intended to be the sequel to Fast Times at Ridgemont High, but securing the actors given their new-found fame was difficult. Chris Penn reprises is brother's "role" as Spicoli, Eric Stolz plays Brad Hamilton, it is believed. Some say "Rat" though. And Lea Thompson definitely plays "Stacy Hamilton". A lot of people say the character of Eileen played by Jenny Wright sounds a lot like Linda Barrett. I kind of hear that.

Yes, you're crazy