While creation of case run an agent with the interval you need and call DoSave activity

What if user opens an existing work item he worked previously?

This can be achived using many uses. I think you can try this approach:

You can have a javascript function on the client which executes for every N seconds(you can googlein for this code). and call OOTB Save UI action by programatically clicking on this hidden save button(you can have save button in the UI). If you are on legacy version of Pega, you can write your own activity which does the job of Save and reload harness functionaliy. If you have 718+, you can check the code of Save UI action and replicate the same in legacy versions. 

I would suggest not to try any approach where you are manually acquiring the locks until you find a better way(There is actually a machanism called nested transactions which will do the job, but i don't think that concept needs to be used here, in fact above approach is simple and you can implemnt in less time).

Its good requirement for specific kind of application. This can be suggested as platform enhancment.