Shawn
Your post is spot on for a 300, having said that I find you need more than 500fpm rate of descent. The buffeting you talk about I think of as a loss of translatinonal lift, the pitching rolling can be the student not getting the ac into a slow speed much like hover taxying down wind. ( assumption we are flying downwind)
At a 500 fpm rate of descent the 300 will power out of the descent by using collective. This tells me that the ac has not " caught up" its own downwash. To actually achieve the symptoms of vortex ring I find you need more like 800 fpm at which point things do become more interesting.
Personally I think this subject ( lesson 15) is very badly taught in this country with instructors not really being shown waht it is