Max, to answer your question, I don't know about the "writing" part, I remember reading that Charlie wrote everything and taught it to Scott and Dan. I've also read that before Dan was ousted, Charlie WAS writing the solos and thought "why am I writing all the solos when it's Dan's job??".
About the founding memebr part......I know that they had a lead before Dan. I remember that Scott was looking for a new lead player and Dan's buddies told him to talk to Scott (Dan worked at a local music store where Scott'd go for gear) because they were into the same music and the rest is "history" as they say. So that answer is no he wasn't a founding member, but he was with Thrax for over a decade. Try the Madhouse a fan page. It has the complete history from the days with Lilker and Turbin to Bush. It's a link on the site, check it out.