I’ve just been reading your piece dated 10 October 2008 on Garratt Pivots and I have some questions:
1. Do the pivots take the whole weight of the boiler unit;
Yes they do. It's nopt that heavy to be honest, I try to get most of the weight in the engine units.
2. Does the boiler unit “balance” on these pivots alone or are there other secondary bearing surfaces or restraints ;
The female pivot is a top hat bearing so provides a modest extra bearing surface. The streatcher that the male half is fitted to sits on this.
3. Does the bearing itself accommodate the relative motion of the bogies on poor track work and if so how much tolerance do you allow, (eg front bogie on a slope, rear bogie still on the level or front bogie canted over with rear bogie still level).
Nothing sophisticated I'm afrid. There is a bit of slop in the bearing and that seems to be enough to accomodate track irregularities. If the track is very bad then this might not work.
I’m thinking about making a model but I want to be clear about the pivots and anticipate potential problems.
Good luck. I suspect you are over-thinking the problem. On a model, these parts don't need to be very sophisticated at all.