The one that you made this a duplicate of does not include anything about them randomly turnimg on. It does not have info about some clocks working and some not.
Creepers should only explode if you punch them or otherwise attack them in anyway. Other wise they should ignore in creative no matter what difficulty
herp derp. I forgot. Retested and it did leave blocks.
I tried that and a wither didn't spawn at all. The blocks just stayed
I had the same thing happen with some NPC villages in super flat. Half of the some of the houses were invisible and right clicking on them with a block updated them.
Using a clock that uses a 1 tick pulser from torches will cause a dispenser not to fire. When using a traditional 2 repeater method does work for me.