Any question answered...

Let's say, two bikes, same speed, same weight (weight saved by smaller wheels is compensated by riders belly fat)
So yes, the smaller wheel would turn faster. And thus be harder to stop. But it's lighter and thus easier to stop.
I guess the difference is negligible, but it's interesting nonetheless...

Think of it on a grander scale and conservation of energy.

(You can ignore the rotational momentum of the wheels, just think how easy and quick it is to stop a wheel that is spinning fast (not in contact with the ground) with the brake, you can even just grab the wheel with your hand to stop it. That's nothing compared to slowing down 100kg of rider+bike from 20mph.)

So you're left with the job of the brake converting the kinetic energy of the bike+rider into heat via friction, which will be the same whether the rim is 28" or 20". This is assuming you're using the same pads on both rims and the contact area of pad/rim is the same for each.

Larger rotors on disc brakes increase the available surface area (larger rotors often have larger pads) so braking performance also can be improved this way. This is akin to using larger pads, not changing the rim size. The increased rotor surface area is also better for heat dissipation as the heat generated is spread over a larger area.

For long descents (e.g. mountains like the Alps) you'd prefer larger/thicker/heavier rims to provide a higher specific heat capacity and avoid the rim heating up enough to blow the tube (it happens!). Tandems especially suffer from this (even with some long descents in the UK) and you'll find they often have a rear hub containing a drag brake to spread the braking load.