The Landcruiser kit won't work on mini because the axle housing ends (bolt pattern) are different. That kit isn't much different from Sky's kit anyway.

Yes, the semi-floating axle does walk in and out a bit. Worse as the bearings wear of course. I'm pretty sure the FSM has a spec for how much play is allowed. Full-floating hubs don't walk because the tapered roller bearings are preloaded against each other. However, lots of people run disks w/ semi-floaters with success, so it must not be THAT big of a problem.

Disk brakes have a WORSE mechanical advantage compared to drums because the drums are self-actuating, as was mentioned above. They requre both more fluid and more pressure. The manual prop valve is needed to increase the pressure to the disks compared to what the drums needed (and for tuning).

Residual valves can be used to help two things- they can help keep slight pressure on the pads so that they aren't pushed back into their bores as easily when the axle walks (as was mentioned), and they also help to "preload" the seals, which means they don't need as much initial fluid to start doing the work (improves pedal travel/feel). Its kind of a bandaid fix, but it seem to help.

There are some aftermarket rear disk kits that use a mini-drum e-brake in the rotor hat like some of the OEM's do. I'm sure they're expensive so I haven't looked into them much to see if there is an application for a Toy.