A misconception. Disney has never stated that this is an on site benefit. They characterize it as a benefit that comes with park admission. During this pahse of the testing, on site guests get to use the pre-booking option. Eventually, everyone will. So it is not a well though-out business move for the reasons you suggest. Perhaps for other reasons, but not these. (And it stands at 60 days, not 6 months).
No. The app functionality is not open to off site guests yet.
Once upon a time I was hopeful. But seeing how much of the ride capacity is now being doled out through FP+, I am not sure that they have left themselves any more room to unlock the tiers or give out more FPs. The rides can only accomodate so many people, and at 75% FP+ ride capacity for some rides, there is nowhere else to go.
Disney has already said that AP holders will have pre-booking options, and the
MDE is giving offsite people future dates when they can pre-book. Granted, these future dates keep rolling forward with each new calendar day. But there would be no reason for them to have created this functionality if they did not intend to activate it some day. The kiosk and i-Pad lines were untenable during the "7-9" crowd days over President's Weekend. Come Easter and the summer months, people will not tolerate that same mess. They have to and will allow pre-booking for offsite guests. Maybe not the same 60 days. Buy they cannot staff the parks with enough CMs and I-Pads to accomodate everyone. Lines over President's Day to use kiosks were an hour long. Will people at Easter tolerate 2 hour lines just to get FP+ scraps? Hardly. So you will see that change.