I’m having trouble following as there at least 2 if not 3 issues being discussed.
In regards to Topics, in the context of deleting / undeleting topics Discourse considers the OP as the Topic. That is, the topic is the first post.
In other words, if the first post is deleted (the trash can icon), the topic is deleted. Regardless of how many replies it might have.
Conversely, if the first post has been deleted, the “counter-clockwise circular arrow” icon can be used to undelete the OP, thus in effect undeleting the topic.
As far as “really” deleting a topic, as in removing all trace of it from the database. I don’t know, but I would strongly suggest this not be attempted.
There is a lot of cross-indexing of tables and unless there is some type of DELETE CASCADE that I haven’t seen yet things could break in the most horrible way imaginable.