Hi,
deleting Depreciation history records will not have any impact on JE already created. That means you will still have amounts in GL Impact for that FAM.
Deleting FAM will be nightmare for you. Manual deleting some FAM background records (values, depreciations...) is really hard.
It is probably better to try with "inactivate", "don't include in reports" for this FAM
I had similar experience, and now when a client start talking "I have a problem with FAM 🙂 ...." call the NetSuite support is my answer