According to what you have for your general forum description is a couple of problems. Guests can't see the topics.
And I assume a problem with threaded vs flat mode.
For the guest issue I would check
Control Panel » Group Management » Forum Permissions
There are two settings "can see forum" that looks like it is okay. But it should have a value of 1 for each forum you want enabled for guests.
The second setting to the right "Can read topics"
Should have a value of 1 for each forum you want them to see.