There are some good videos on workflows, user inputs and variables from Kevin.
Hope it helps for the first set of queries above.
>>how to get rid of entries in this table (I think there is a variable waiting for userinput of which I cannot find the source)?
We understand this issue. For now, you will have to manually search in the workflow as to where you have used $UserInput. If '$' has been used in the workflow design, then it automatically becomes a user input.
In the upcoming release of WFA (2.1 version), we will be providing a search feature where a user input can be searched inside a workflow and eliminated or modified as required.