Floor sagging between joist- please help
The given parameters are that the floor joists and the flooring are good, but the sub-floor has "received significant damage." The solution then, is to replace the sub-floor. Don't assume that removing the existing hardwood floor will destroy it. Re-using old hardwood flooring is common. It reads like the hardwood and the sub-floor have already separated. With a 100 year old home, there probably isn't any glue (construction adhesive) to deal with, just nails that have lost their hold. An added benifit is that after removing the bad sub-floor, you could shim up under the replacement to correct the leveling issue.
A few floor boards could be damaged during the removal. And the removed hardwood needs to be prepped before re-installation. But those issues are part of a good floor restoration project. With a bad sub-floor, it is hard to imagine a practical or quality alternative.