i always sink into the saddle more and just slow my movements more and just make it feel like your just tryin to hold everything together. i have never heard the way your first instructor said
Never make your seat heavy though, there is a difference between sinking into the saddle and sitting into the saddle... if you sink into the saddle, you deepen your seat and you can just continue in sitting trot, if you want to walk then start to move your hips in a more walky way (if that makes sense).
the latter would hollow the horse but your boy could just be resisting the transition anyway. Just make your body soft in the transition and do lots of transitions so they aren't so stressful (not that i'm saying your horse is stessed