Isn't that Garry Marshall playing drums? I heard he was a drummer.

yep. he just couldn't resist.

and Fred Fox appears as a singer in that episode where Joanie and Chachi's song gets stolen (by Fred's character no less)

Well, at least Hallmark showed the Graduation episodes. I do have a question, though. According to the digital guide, it showed that episodes 17 and 18 were the Graduation episodes and 19 was the Army recruiting episode and 20 was when Joanie wanted to be in the She-Devils. Wouldnt the graduation episodes be the last episodes of the season?

Wouldnt the graduation episodes be the last episodes of the season?
No. Six episodes aired after the "Graduation" episodes in season four.