Yes when u have old and new data mixed together the old SStable will not be dropped until new SStable is fully expired.
The single sstable compaction has quite strict checks for whether it should be started, this option disables those checks and for some use cases this might be needed. Note that this does not change anything for the actual compaction, tombstones are only dropped if it is safe to do so - it might just rewrite an sstable without being able to drop any tombstones.