The issue was discussed privately. I determined that Edit Post Date by Waindigo was causing a template modification compilation error for several different add-ons, and that Poll Permissions by AndyB is incompatible with my add-on. With both of those disabled, Slide Out Poll works correctly.
I do not know what the problem is with Waindigo's add-on, but I don't have any intentions of making a compatibility fix for AndyB's add-on. However, I think the application order for the template modifications can be changed so that my add-on goes first and AndyB's goes second and then they don't conflict. I'm not sure.
The "Add a Poll" button now toggles to "Cancel Poll" once the poll options are displayed. If you click the "Cancel Poll" button, the poll text will be wiped out and the poll controls will be hidden again.
Also made superficial changes to admin options phrasing.
For the issue of duplicate "create thread button" and I understand having 2 is confusing and unprofessional, while having nothing at very bottom isn't good either, what about removing "create thread" button from first row and placing it in second. Then you will always have it at bottom, whether poll or not, and not have a duplicate? The same with "preview" too. And putting cancel poll at very bottom if poll is selected.
There's no reason to have the create thread button above further options like "watch this thread" etc