There is no such thing as a multi-dig per Inno rules, although Inno does 'discourage' you from digging too close together or solely for the purpose of annoyance. I have never really figured out what "too close" means to Inno as every pronouncement is vague to me. It is like playing football without knowing the exact rules and the referee isn't going to tell you what they are when you ask beforehand - but don't transgress one or else.
The "multi" rule (and alternate digs as well) is a self-imposed one between alliances and they differ in detail from world to world. We can argue about the merits, or lack thereof, with these. My opinion is that they are fine in principle but they soon get bogged down in pettiness. e.g. Your dig is a multi-dig as it is 7hrs 59 mins after the prior ff, and not 8 hrs + (really). Worse still, you run into players who insist that the rules in world A should apply on world B, even if there is no agreement whatsoever on world B.
So Philo, I am in your camp in a qualified way! If there is no agreement between your alliance and that of the fort owner, dig whenever you like provided it is say 4 hrs after the prior ff (not unreasonable I think with present regen gear and craftable health buffs). I don't see why you should be constrained by agreements between alliances other than yours.
As for this ff on Friday on all worlds, what is the purpose on most of them? You are not going to fill them even under the old quotas, most of them nowhere near. Please, why all at the same time and why on a Friday (@ 11:00am for me)? I thought the consensus here was to have them on a weekend with even the CM saying that. What is the purpose of this forum then if something as simple as that is going to be ignored?