thenorm, you stated
'i dont believe that a very slight toe out will cause the wobble.'
You should drive a double positrac four wheel drive with toe out and see just how creepy toe out under power is on front end, the vehicle constantly darts one way and then the other with no warning. One of the most unstable setups there is, terrifying to drive. The tires try to pull car in two different directions.
With toe out the tires will constantly be fighting each other to go opposite ways, you may not see it on dry pavement but that's exactly the type of scrub and slip that could easily untrack car on very substandard conditions like ice. Both toe in and toe out scrub tires but toe in scrubs and both sides pretty much slip evenly as they are scrubbing. Reason why is that both tires are trying to come together, if suspension tight the slip is gradual and evened out, the tires can only drive against each other so far before they HAVE to slip and keep on doing it. Toe out more like one tire scrubs a bit then slips when other tire yanks it back. An alternating slip-slip-slip as the two tires fight with each other. May not be dead even one for one trade either. They trade slip because tires are moving away from each other and at some arbitrary point one gives in to the other. The car weight trying to yank tire back straight in line (think of a trailer) adds to it too. The scrub/slip at same time does much less to untrack car in ice than the alternate scrub and slip. Slip in second case is worse because it accumulates before relief rather than being dissipated gradually.
Basic difference..........toe in, tires can both be out of dead straight but their combined motion can still result in dead straight tracking, toe out cannot do that, one tire goes one way and the other one goes its separate way, no way to make them work together. The skittishness from that makes hell on ice, or rain for that matter. You won't see slight amount affects unless you lose control of car too easily, that's it showing up.
Ford's reason for more weight increasing toe..................designed in for safety.
By 'scrub' I mean rubber rolling not dead straight, the friction is at an angle and not in line with tire sides. It produces the feathering on tread blocks.
By 'slip' I mean any giving of tread to neutralize distorting forces, the tire could actually slip on road surface (in water or on ice) or tread blocks can squirm to relieve that scrub force. Yes, a very small amount occurs on dead straight normal tires. A small amount allows tread to fit into road irregularities for better traction but more can be bad. It kills tire life past a certain point.