Hmmm…
Regarding the DOIs migration/upgrade: it seems like something went wrong with the upgrade. They should have been migrated with this code line: ojs/upgrade.xml at ojs-2_4_8-3 · pkp/ojs · GitHub.
In 2.3.6 the DOIs were saved in a column in the DB table articles. Since 2.4.0 they are saved in article_settings table. Thus, you should see entries with setting_name = “pub-id::doi” in your DB table article_settings. Is this not the case?
When you create a review form and activate it, you should be able to chose that form for a review assignment, using the link “Select Review Form”.