ST,
You have options.
If you leave room for text boxes by having a large right margin, you can add a small, just right size, box for each comment. That was what I was getting at. It can be tedious adding and positioning those boxes, and I suppose that's what you might object to.
I hate this second idea, but I'll put it out there anyway. One skinny page-length text box on the right, filled with Return characters. Drop your Text Insertion Cursor wherever you want to make a comment. Delete as many Return characters as you displaced in making your comment or your column of returns will outgrow your column space.
Thirdly. A two column main text layer, big column on left, smaller on right. Do all your content authoring on the left and Insert > Page Break when you get to the bottom of the page so your content doesn't automatically flow to the right hand column. At a point just above where you would like to make a comment on the right side, place the text insertion cursor in the text on the left and Insert > Layout Break. Also Insert > Layout Break just below the section you are commenting on. Just above the second Layout Break, Insert > Column Break. Type your comment into the Right Column, where the cursor will have moved to.
The fourth option is the table. Some don't like the table option because it requires that you start a new row before you reach the page bottom margin. If you don't, all the current cell's content will jump to the next page when you run out of space at the bottom of the page.
Jerry