The お稽古, especially during seminars, also has its rules akin to the 点前 and the seven exercise. These are almost harder still, because they can differ from teacher to teacher.
Generally, you want to make the teacher wait as little as possible.
With the sweets, there are two ways:
One models the
茶事: You bring in the sweets, then you set up for
濃茶 as the guests are eating them. This is the way many students of MN do it.
The other one invents a new mode: You set up the
点前, then you bring in the sweets. After the 総礼 with the
蓋置, the guests eat the sweets.
If you need to excuse yourself during a
点前 to prepare, take your fan, and announce 「ご準備をさせていただきます」