To see if members can delete their own posts / threads you need to look through your permissions.
As your forum is locked down I cannot view anything apart from the front page, so here is an idea for missing threads, go to control panel > default settings and check the first line,? When users enter a category I would like them to see the last (number box) Days of threads has 9999 in it and then submit.
If it has a lower number threads can just go missing after a set time limit and it is best to have 9999 in the box.
Also check the next three lines and make sure you have at least 10 in each drop down choice, I have 30 on each of mine, always remember to submit after changing something.
Regarding uninvite guests asking to join your forum, there isn't much you can do about that, I have just done a test join on your forum to see what settings you have, you can just delete my application via control panel > inviting.
I see you have it set for approval but no message , you can set it so people have to include a message on application by going to preferences > scroll down to Member #3 and choose Members need to leave a message and get approval before joining.
Let me know if that helps or if you still have missing topics, you can if you want let me be a member, add me as an admin and I will check your settings / permissions if you want, I will then delete myself from your forum that is up to you and only if you need help with that.
I have done that with 100's of forums over the years but the choice is yours.
Hope that all helps.
