Hi krishna,
1. Yes, you can chagne rule type from over write to summation then your dso becomes addivitve.
Need to for all key figures.
2 You can read the below doc. will be helful.
From psa to DSO once you got data, then while loading from dso to cube no need to worry about delta types. When you want load full load from dso to cube, load data from active to to inf cube.
When you load delta fromd so to cube, you can load from chnage log table.
3. Yu can check shared doc, it may answer you.
4. Yes
Thanks