I think people are making this too complicated. On-site guests and AP holders will be allowed to pre-book experiences--this is the perk to staying on-site or buying an AP.
Off-site guests will essentially use FP+ the same way FP- is used right now. Remains to be seen, obviously, how well that works, but I don't see it as being that different from the way things work now, except that, presumably, on-site guests and AP holders will have first dibs on FP slots. However, one of the great things about this is that the system should not be as linear, and FP times may be scattered throughout the day rather than becoming later and later until they run out as they do now. I could see this being used to keep a more even flow of guests at FP+ attractions. IOW, a certain number of slots will always be "held back" for day-of use by all guests. But if a bunch of people pre-booked FP+ times for evening leaving a lot of afternoon slots, the kiosks will show a lot of availability in the afternoon. And obviously this will change throughout the day as people make changes on the fly.
I think now that the intention has never been to limit on-site guests to 3 FP+ with tiers, and that guests will still be able to make additional day-of FP bookings. Maybe there won't be any limits on FP on the day-of except overall availability (like now) and not being able to "hold" more than one FP+ at a time.