a - Should the marginal illustration be inserted on the line immediately following the lines where the words "Figure 2" and "Figure 4" appear in the text, or should it be inserted at the end of the completed sentence?
I would place the figures at the end of the paragraph in which they are cited. That's better than interrupting the text.
The captions for the figures (FIGURE 1, FIGURE 2, etc) would be in 7/5 as captions normally are. Tactile Graphics Guidelines propose that graphics be placed at the left margin for ease in finding them. Do not place your graphic so its left margin is indented as marginal material. No reference indicator is required for captions.
Should the margins for this note be cell 5 (heading) "Looking Ahead to Calculus" and 7/5 for the text portion of the note?
If you are going to make the decision to use cell 5 headings for a certain kind of marginal note (such as "Looking Ahead to Calculus") then just format the text following the heading as you would normally format it -- the paragraph would start in cell 3 with runovers in cell 1 and displayed items in 3/5.
If you want to treat this material as marginal material, with a general reference indicator (not a literary one) then your note will begin in 7 with runovers in 5, and the diplayed would have to be 7/9. In this event, the heading would probably have to become a paragraph heading on the same line as the following text. Be sure to look through your book to determine if there will be any comparable marginal material that could present a problem with your chosen format.