If I'm understanding the requirements correctly, the event is required to occur before the records can be destroyed, and the records must be held for at least six years, but the timing of the event has no bearing on eligibility for destruction?
Since eligibility is going to be based on the filing date, you might try creating two retention schedules, one "destroy after six years" (with a 72 month hold period) and another "hold indefinitely" (with no final disposition specified, hold period optional). Then the first one can be set as an alternate retention schedule for the first one, with the event that's required for destruction configured as the trigger to change over.
Then the first schedule gets set up as the retention schedule for the records as they enter the system. That way, the records will be held indefinitely (per the first schedule) but when a date for the changeover event is given for them they'll swap over to the "destroy after six years" schedule, with the six years measured from their filing date.
You shouldn't need a cutoff instruction in this scenario (as of 10.4 or thereabouts the system doesn't require one).
If I've understood the requirements correctly, I believe a setup like this would handle them.