Thanks for the report, let's try to break it down in separate reports so that it gets actionable.

(In reply to John McClure from comment #0)

1. Special:PagePreparation does not prefill its pagename field when it's

been called as Special:PagePreparation/fullpagename or as
Special:PagePreparation?title=fullpagename. If it would be, then a button
can be added to the Toolbox to initiate the translation process. The current
process requires re-entry of the pagename on the special page.

19.00 < Nemo_bis> hypergrove: I asked it at https://bugzilla.wikimedia.org/show_bug.cgi?id=69273#c1

2. Special:PagePreparation does not give an error message when the pagename

entered is for a non-existing page.

Apparently not filed, only bug 69796.

3. When a page is reverted to its pre-preparation state, the page still

shows in Special:Translate or special:LanguageStats... the point being that
there seems no way to "back out" a page from translation process once it has
entered it.

That's not about Special:PageMigration but it's covered in the last section of https://www.mediawiki.org/wiki/Help:Extension:Translate/Page_translation_administration#Life_of_a_translatable_page
A patch is pending: https://gerrit.wikimedia.org/r/#/c/161931/4

4. A new section in the sidebar would be nice concerned only with

translation, eg it lists all the tools/special:pages available to the user
for that page. In particular, it could show (a) options such as "Prepare",
"Nuke translations" and so on

That's really too much stuff for the sidebar, not going to be piled there (bug 43170).

(b) current status of the translation process
for the page, eg Prepared for translation, Marked for translation, Partially
translated, Fully translated, and Outdated translation.

This is already in the header, you may have forgotten some step of https://www.mediawiki.org/wiki/Help:Extension:Translate/Page_translation_example

It would be nice to
have a switch in LocalSettings to display such toolbar. Currently, naive
users are confused who want to go from page->translation process; the
current design is always to go translate->page.

I don't understand what this means.

That's why I think a
Translation section in the sidebar of translateable pages (not special
pages) would be very useful for the whole community.

https://gerrit.wikimedia.org/r/#/c/203389/ fixes point number 1.

Patches for #1 and #3 have been merged; #4 is covered in T45170; this leaves us with #2. (Please do not report several issues in one task.)