Brief Solution to The Mystery of the Added Slip of Paper

Why is there a sewn-in slip of paper on MS A? Why doesn't the poet just write the lines out?

Because it was a rare, surviving earlier piece of text already copied out. Rather than write the lines out again Wordsworth decided to sew it in here.

What is the order of entry of the text onto this page?

Order of entry is actually straightforward, although the reworkings at the bottom of the page make it seem more complex than it is. The text is first entered to the line count of 377 (with a gap left for the word "Valley"). Next the slip is sewn in, leaving the space beneath it blank. Wordsworth then continues under the slip, starting at line 398 "But not betray'd by tenderness of mind" and perhaps writing down to the second "400" line count, or to the bottom of the page. Thirdly, the poet reworked the top line under the slip, on the base text, (these lines may also have been unresolved on the slip before it was torn off). At some point he also added the line and the word "Valley" at the top of the page. This threw the line count off.

How many different times of entry are there on the page?

It's hard to be sure, but a possibility would be:

1	Copying at top of page
2	Sewing-in of slip (written earlier)
3	Writing down to bottom of page
4	Entry of first line-count up to crossed-out "400"
5	Correction of bottom section
6	Addition of line above slip.
7	Change to line-count with first "400" added.