Hi, my two cents:
1. This does not seem to be supported, but why would you want to do this?
If you simply want your changes to affect pending order, make those changes in the current cartridge, in most cases this is possible.
In the same cartridge, you can for example define a new version of the process and then move your orders by a process exception to a position in the new process.
2. Depends on what exactly you want to achieve.
AFAIK, the task expected completion is used for two things: jeopardy and perhaps worklist sorting.
For jeopardy, you can write custom logic in your rule.
For sorting, you could populate a flex header at task start e.g. by a task event and use that for sorting / display.
3. Do you want to see all tasks yet to be processed each with its expected completion cumulative?
I think this is difficult to achieve since typically process definition contains alternative paths to take you from any given point to completion.
And you don't know with what exit status each task will complete.
At any rate, this would be a 100 percent custom report, the DB is all yours, good luck.
4. There certainly is a way, if you write custom logic to convert those files to the design studio format. Just a matter of coding.