In future please only ask one question per post.
面白い事件が起こりました
An interesting incident occurred.
The verb here is 起こる, which is an intransitive verb meaning "something happened". If you want to say that you caused the incident you need the transitive version of the verb which is 起こす. Your sentence would then be 面白い事件を起こしました (I caused an interesting incident). Note the change from が to を.
窓側の席のほうがいいですが
I would prefer a window seat
The が at the end literally means 'but'. The rest of the sentence is left unsaid. You can fill in the missing part in your head with " but I don't want to be a bother" or anything like that. This is a very common way to make a request in Japanese. It makes the request less direct and therefore more polite. I feel sure this has been discussed a lot on this site but this is the only link I could find.
石鹼はそこに置いてあります
The soap is there.
There is no imperative form in this sentence. This is the て form of the verb with the auxiliary ある. The てある grammar point is discussed in this link.