What about outdoor use? Any problems with rust or corrosion from rain, snow, etc?

We've had ours since August outdoors. No problems with rust or corrosion on the rotational device, however, the carabiners developed a bit of rust on the part where you screw them closed. We wiped them clean when we realized it and sealed them. No permanent damage and they still worked properly.

What is the rating on the carabiners?

They are not officially rated or marked. They are made of steel, and their quality is somewhere between a cheap Home Depot keychain carabiner and an actual rated carabiner (the gate area is thinner than the body). IMO, They are probably fine for a static load like hanging a hammock, but I would not use them for anything subject to dynamic forces (I.e. aerial silks). The swivel on the other hand is good quality, pinned and the thrust bearing feels nice both free and under heavy tension.

What is the working load capacity on the carabiners?

They are not officially rated or marked. They are made of steel, and their quality is somewhere between a cheap Home Depot keychain carabiner and an actual rated carabiner. IMO, They are probably fine for a static load like hanging a hammock, but I would not use them for anything subject to dynamic forces (I.e. aerial silks). The swivel on the other hand is good quality, pinned and the thrust bearing feels nice both free and under heavy tension.

I'd like to use this with a tree swing giant 40'' saucer swing. Is this designed to have two carabiners hanging from the bottom portion?

Absolutely not! This is the difference between a swivel with a triangular eye like this one (single point of attachment) and a round eye (which can support two carabiners). This one can be adapted using a shackle, delta link or other suitable rated hardware to safely combine two carabiners exerting force in multiple directions to the swivel.