thanks for your feedback. About your questions:
1) Unfortunately, I cannot troubleshoot your problem just from your phrase "it imported only 7 comment subscriptions on almost 300" ;) I need further details about your data to see what the problem could be. The plugin itself performs the following query to import the old data, and it has worked fine on most installations so far
INSERT INTO $this->table_subscriptions (email,status,post_ID,dt)
SELECT DISTINCT comment_author_email, comment_subscribe, comment_post_ID, comment_date
WHERE comment_author_email LIKE '%@%.%'
But I understand your concerns, so starting from version 1.3, the old data won't be deleted anymore. This will allow users who are not satisfied with my plugin to rollback to the other one in just a few clicks.
2) I tested my plugin with TwentyTen, and the only thing I see is 'comments are closed for this post', which is what should happen, indeed. I'm looking into it, though, and I'm trying to completely disable comments for the management page.